Light-emitting element

ABSTRACT

A light-emitting element having high external quantum efficiency is provided. A light-emitting element having low drive voltage is provided. Provided is a light-emitting element which includes a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes. A combination of the first organic compound and the second organic compound forms an exciplex (excited complex). An emission spectrum of the exciplex overlaps with an absorption band located on the longest wavelength side of an absorption spectrum of the phosphorescent compound. A peak wavelength of the emission spectrum of the exciplex is longer than or equal to a peak wavelength of the absorption band located on the longest wavelength side of the absorption spectrum of the phosphorescent compound.

TECHNICAL FIELD

The present invention relates to light-emitting elements using anorganic electroluminescence (EL) phenomenon (hereinafter suchlight-emitting elements are also referred to as organic EL elements).

BACKGROUND ART

An organic EL element has been actively researched and developed. In afundamental structure of the organic EL element, a layer including aluminescent organic compound (hereinafter also referred to aslight-emitting layer) is interposed between a pair of electrodes. Theorganic EL element has attracted attention as a next-generation flatpanel display element owing to characteristics such as feasibility ofbeing thinner and lighter, high speed response to input signals, andcapability of direct current low voltage driving. In addition, a displayusing such a light-emitting element has a feature that it is excellentin contrast and image quality, and has a wide viewing angle. Further,being a planar light source, the organic EL element has been attemptedto be applied as a light source such as a backlight of a liquid crystaldisplay and a lighting device.

The emission mechanism of the organic EL element is of acarrier-injection type. That is, by application of voltage with alight-emitting layer interposed between electrodes, electrons and holesinjected from the electrodes are recombined to make a light-emittingsubstance excited, and light is emitted when the excited state relaxesto the ground state. There can be two types of the excited states: asinglet excited state (S*) and a triplet excited state (T*). Thestatistical generation ratio of the excited states in a light-emittingelement is considered to be S*:T*=1:3.

In general, the ground state of a light-emitting organic compound is asinglet state. Therefore, light emission from the singlet excited state(S*) is referred to as fluorescence because it is caused by electrontransition between the same spin multiplicities. On the other hand,light emission from the triplet excited state (T*) is referred to asphosphorescence where electron transition occurs between different spinmultiplicities. Here, in a compound emitting fluorescence (hereinafterreferred to as fluorescent compound), in general, phosphorescence is notobserved at room temperature, and only fluorescence is observed.Accordingly, the internal quantum efficiency (the ratio of generatedphotons to injected carriers) in a light-emitting element including afluorescent compound is assumed to have a theoretical limit of 25% basedon S*:T*=1:3.

On the other hand, when a compound emitting phosphorescence (hereinafterreferred to as phosphorescent compound) is used, an internal quantumefficiency of 100% can be theoretically achieved. That is, higheremission efficiency can be obtained than using a fluorescent compound.For these reasons, a light-emitting element including a phosphorescentcompound has been actively developed in recent years in order to achievea high-efficiency light-emitting element. As the phosphorescentcompound, an organometallic complex that has iridium or the like as acentral metal has particularly attracted attention owing to their highphosphorescence quantum yield; for example, an organometallic complexthat has iridium as a central metal is disclosed as a phosphorescentmaterial in Patent Document 1.

When a light-emitting layer of a light-emitting element is formed usinga phosphorescent compound described above, in order to suppressconcentration quenching or quenching due to triplet-triplet annihilationin the phosphorescent compound, the light-emitting layer is often formedsuch that the phosphorescent compound is dispersed in a matrix ofanother compound. Here, the compound serving as the matrix is calledhost material, and the compound dispersed in the matrix, such as aphosphorescent compound, is called guest material.

REFERENCE Patent Document

-   [Patent Document 1] International Publication WO 00/70655 pamphlet

DISCLOSURE OF INVENTION

However, it is generally said that the light extraction efficiency of anorganic EL element is approximately 20% to 30%. Accordingly, consideringlight absorption by a reflective electrode and a transparent electrode,the external quantum efficiency of a light-emitting element including aphosphorescent compound has a limit of approximately 25% at most.

Further, as described above, application of organic EL elements todisplays and lightings has been considered. One of objects to beachieved here is a reduction in power consumption. In order to reducepower consumption, it is required to reduce the drive voltage of theorganic EL element.

An object of one embodiment of the present invention is to provide alight-emitting element with high external quantum efficiency. Anotherobject of one embodiment of the present invention is to provide alight-emitting element with low drive voltage.

Note that the invention to be disclosed below aims to achieve at leastone of the above-described objects.

One embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex, inwhich an emission spectrum of the exciplex overlaps with an absorptionband located on the longest wavelength side of an absorption spectrum ofthe phosphorescent compound, and in which a peak wavelength of theemission spectrum of the exciplex is longer than or equal to a peakwavelength of the absorption band located on the longest wavelength sideof the absorption spectrum of the phosphorescent compound.

Another embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex, inwhich an emission spectrum of the exciplex overlaps with an absorptionband located on the longest wavelength side of an absorption spectrum ofthe phosphorescent compound, and in which a difference between a peakwavelength of the emission spectrum of the exciplex and a peakwavelength of an emission spectrum of the phosphorescent compound is 30nm or less.

Another embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex, inwhich an emission spectrum of the exciplex overlaps with an absorptionband located on the longest wavelength side of an absorption spectrum ofthe phosphorescent compound, and in which a peak wavelength of theemission spectrum of the exciplex is longer than or equal to a peakwavelength of the absorption band located on the longest wavelength sideof the absorption spectrum of the phosphorescent compound and shorterthan or equal to a peak wavelength of an emission spectrum of thephosphorescent compound. In addition, it is preferable that a differencebetween the peak wavelength of the emission spectrum of the exciplex andthe peak wavelength of the emission spectrum of the phosphorescentcompound be 30 nm or less.

Further, one embodiment of the present invention is the aforementionedlight-emitting element in which the exciplex is formed from a singletexciton of the first organic compound.

Further, one embodiment of the present invention is the aforementionedlight-emitting element in which the exciplex is formed from an anion ofthe first organic compound and a cation of the second organic compound.

In the aforementioned light-emitting element, it is preferable thatexcitation energy of the exciplex be transferred to the phosphorescentcompound, so that the phosphorescent compound emits phosphorescence.

In the aforementioned light-emitting element, it is preferable that atleast one of the first organic compound and the second organic compoundbe a fluorescent compound.

In the aforementioned light-emitting element, it is preferable that thephosphorescent compound be an organometallic complex.

The light-emitting element of one embodiment of the present inventioncan be applied to a light-emitting device, an electronic device, and alighting device.

According to one embodiment of the present invention, a light-emittingelement having high external quantum efficiency can be provided.According to another embodiment of the present invention, alight-emitting element having low drive voltage can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an absorption spectrum and emission spectra according toExample 1.

FIG. 2 shows an absorption spectrum and emission spectra according toExample 1.

FIG. 3 shows an absorption spectrum and emission spectra according toExample 1.

FIG. 4 shows an absorption spectrum and emission spectra according toExample 1.

FIG. 5 illustrates a concept of one embodiment of the present invention.

FIG. 6 illustrates energy levels of an exciplex applied to oneembodiment of the present invention.

FIGS. 7A to 7C each illustrate a light-emitting element of oneembodiment of the present invention.

FIG. 8 illustrates a structure of a light-emitting element of Example 2.

FIG. 9 shows voltage-luminance characteristics of the light-emittingelement of Example 2.

FIG. 10 shows voltage-current characteristics of the light-emittingelement of Example 2.

FIG. 11 shows luminance-power efficiency characteristics of thelight-emitting element of Example 2.

FIG. 12 shows luminance-external quantum efficiency characteristics ofthe light-emitting element of Example 2.

FIG. 13 shows an emission spectrum of the light-emitting element ofExample 2.

FIG. 14 shows results of reliability tests of the light-emitting elementof Example 2.

FIG. 15 shows a relationship between a peak wavelength of an emissionspectrum of an exciplex and a HOMO level of a substance X according toExample 3.

FIG. 16 shows a relationship between a peak wavelength of an emissionspectrum of an exciplex and external quantum efficiency of alight-emitting element according to Example 3.

FIG. 17 shows calculation results according to one embodiment of thepresent invention.

FIGS. 18A1, 18A2, 18B1, 18B2, 18C1, and 18C2 show calculation resultsaccording to one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described with reference to drawings. Note that theinvention is not limited to the following description, and it will beeasily understood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe invention. Therefore, the invention should not be construed as beinglimited to the description in the following embodiments. Note that inthe structures of the invention described below, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and description of such portions is notrepeated.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described.

The light-emitting element of this embodiment includes a light-emittinglayer containing a guest material as a light-emitting substance, a firstorganic compound, and a second organic compound. Specifically, aphosphorescent compound is used as the guest material. Note that one ofthe first and second organic compounds, the content of which is higherthan that of the other in the light-emitting layer, is called hostmaterial.

The structure in which the guest material is dispersed in the hostmaterial can prevent the light-emitting layer from crystallizing.Further, it is possible to suppress concentration quenching due to highconcentration of the guest material, and thus the light-emitting elementcan have higher emission efficiency.

Note that in this embodiment, it is preferable that a triplet excitationenergy level (T₁ level) of each of the first and second organiccompounds be higher than that of the guest material. This is because,when the T₁ level of the first organic compound (or the second organiccompound) is lower than that of the guest material, the tripletexcitation energy of the guest material, which is to contribute to lightemission, is quenched by the first organic compound (or the secondorganic compound) and accordingly the emission efficiency is decreased.

<Elementary Processes of Light Emission>

First, a description is given of general elementary processes of lightemission in a light-emitting element using a phosphorescent compound asa guest material.

(1) The case where an electron and a hole are recombined in a guestmolecule, and the guest molecule is excited (direct recombinationprocess).

(1-1) When the excited state of the guest molecule is a triplet excitedstate, the guest molecule emits phosphorescence.

(1-2) When the excited state of the guest molecule is a singlet excitedstate, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state and emitsphosphorescence.

In other words, in the direct recombination process in (1), as long asthe efficiency of intersystem crossing and the phosphorescence quantumyield of the guest molecule are high, high emission efficiency can beobtained. Note that as described above, the T₁ level of the hostmolecule is preferably higher than the T₁ level of the guest molecule.

(2) The case where an electron and a hole are recombined in a hostmolecule and the host molecule is put in an excited state (energytransfer process).

(2-1) When the excited state of the host molecule is a triplet excitedstate and the T₁ level of the host molecule is higher than that of theguest molecule, excitation energy is transferred from the host moleculeto the guest molecule, and thus the guest molecule is put in a tripletexcited state. The guest molecule in the triplet excited state emitsphosphorescence. Note that energy transfer to a singlet excitationenergy level (S₁ level) of the guest molecule can occur in theory, butis unlikely to be a main energy transfer process because, in many cases,the S₁ level of the guest molecule has a higher energy than the T₁ levelof the host molecule; therefore, a description thereof is not givenhere.

(2-2) When the excited state of the host molecule is a singlet excitedstate and the S₁ level of the host molecule is higher than the S₁ leveland T₁ level of the guest molecule, excitation energy is transferredfrom the host molecule to the guest molecule, and thus, the guestmolecule is put in a singlet excited state or a triplet excited state.The guest molecule in the triplet excited state emits phosphorescence.In addition, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state, and emitsphosphorescence.

In other words, in the energy transfer process in (2), it is importanthow efficiently both the triplet excitation energy and the singletexcitation energy of the host molecule can transfer to the guestmolecule.

In view of the above-described energy transfer processes, before theexcitation energy of the host molecule is transferred to the guestmolecule, when the host molecule itself is deactivated by emitting theexcitation energy as light or heat, the emission efficiency isdecreased. The inventors have found out that when the host molecule isin a singlet excited state (the above (2-2)), the energy is unlikely totransfer to the guest molecule, i.e., the phosphorescent compound, andthe emission efficiency is likely to be decreased as compared to whenthe host molecule is in a triplet excited state (the above (2-1)). Thus,the inventors have focused on that fact as an object. The reason hasbeen found as follows in consideration of a more detailed energytransfer process.

<Energy Transfer Process>

The following describes energy transfer processes between molecules indetail.

First, as a mechanism of energy transfer between molecules, thefollowing two mechanisms are proposed. A molecule providing excitationenergy is referred to as host molecule, while a molecule receiving theexcitation energy is referred to as guest molecule.

<<Förster Mechanism (Dipole-Dipole Interaction)>>

In Förster mechanism, direct intermolecular contact is not necessary forenergy transfer. Through a resonant phenomenon of dipolar oscillationbetween a host molecule and a guest molecule, energy transfer occurs. Bythe resonant phenomenon of dipolar oscillation, the host moleculeprovides energy to the guest molecule, and thus, the host molecule isput in a ground state and the guest molecule is put in an excited state.The rate constant k_(h*→g) of Förster mechanism is expressed by Formula(1).

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack & \; \\{{\, k_{h^{*}\rightarrow g}} = {\frac{9000\; c^{4}K^{2}\phi\;\ln\; 10}{128\;\pi^{5}n^{4}N\;\tau\; R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{4}}{\mathbb{d}v}}}}} & (1)\end{matrix}$

In Formula (1), ν denotes a frequency, f′_(h)(ν) denotes a normalizedemission spectrum of a host molecule (a fluorescent spectrum in energytransfer from a singlet excited state, and a phosphorescent spectrum inenergy transfer from a triplet excited state), ε_(g)(ν) denotes a molarabsorption coefficient of a guest molecule, N denotes Avogadro's number,n denotes a refractive index of a medium, R denotes an intermoleculardistance between the host molecule and the guest molecule, τ denotes ameasured lifetime of an excited state (fluorescence lifetime orphosphorescence lifetime), c denotes the speed of light, φ denotes aluminescence quantum yield (a fluorescence quantum yield in energytransfer from a singlet excited state, and a phosphorescence quantumyield in energy transfer from a triplet excited state), and K² denotes acoefficient (0 to 4) of orientation of a transition dipole momentbetween the host molecule and the guest molecule. Note that K²=⅔ inrandom orientation.

<<Dexter Mechanism (Electron Exchange Interaction)>>

In Dexter mechanism, a host molecule and a guest molecule are close to acontact effective range where their orbitals overlap, and the hostmolecule in an excited state and the guest molecule in a ground stateexchange their electrons, which leads to energy transfer. The rateconstant k_(h*→g) of Dexter mechanism is expressed by Formula (2).

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 2} \right\rbrack & \; \\{k_{h^{*}\rightarrow g} = {\left( \frac{2\;\pi}{h} \right)K^{2}{\exp\left( {- \frac{2\; R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}{\mathbb{d}v}}}}} & (2)\end{matrix}$

In Formula (2), h denotes a Planck constant, K denotes a constant havingan energy dimension, ν denotes a frequency, f′_(h)(ν) denotes anormalized emission spectrum of a host molecule (a fluorescent spectrumin energy transfer from a singlet excited state, and a phosphorescentspectrum in energy transfer from a triplet excited state), ε′_(g)(ν)denotes a normalized absorption spectrum of a guest molecule, L denotesan effective molecular radius, and R denotes an intermolecular distancebetween the host molecule and the guest molecule.

Here, the efficiency of energy transfer from the host molecule to theguest molecule (energy transfer efficiency Φ_(ET)) is thought to beexpressed by Formula (3). In the formula, k_(r) denotes a rate constantof a light-emission process (fluorescence in energy transfer from asinglet excited state, and phosphorescence in energy transfer from atriplet excited state) of a host molecule, k_(n) denotes a rate constantof a non-light-emission process (thermal deactivation or intersystemcrossing) of a host molecule, and τ denotes a measured lifetime of anexcited state of a host molecule.

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 3} \right\rbrack & \; \\{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3)\end{matrix}$

First, according to Formula (3), it is found that the energy transferefficiency Φ_(ET) can be increased by further increasing the rateconstant k_(h*→g) of energy transfer as compared with another competingrate constant k_(r)+k_(n) (=1/τ). Then, in order to increase the rateconstant k_(h*→g) of energy transfer, based on Formulae (1) and (2), inFörster mechanism and Dexter mechanism, it is preferable that anemission spectrum of a host molecule (a fluorescent spectrum in energytransfer from a singlet excited state, and a phosphorescent spectrum inenergy transfer from a triplet excited state) largely overlap with anabsorption spectrum of a guest molecule.

Here, the present inventors have considered that the absorption band onthe longest wavelength side (lowest energy side) in the absorptionspectrum of the guest molecule is important in considering the overlapbetween the emission spectrum of the host molecule and the absorptionspectrum of the guest molecule.

In this embodiment, a phosphorescent compound is used as the guestmaterial. In an absorption spectrum of the phosphorescent compound, anabsorption band that is considered to contribute to light emission mostgreatly is an absorption wavelength corresponding to direct transitionfrom a singlet ground state to a triplet excitation state and a vicinityof the absorption wavelength, which is on the longest wavelength side.Therefore, it is considered preferable that the emission spectrum (afluorescent spectrum and a phosphorescent spectrum) of the host materialoverlap with the absorption band on the longest wavelength side in theabsorption spectrum of the phosphorescent compound.

For example, most organometallic complexes, especially light-emittingiridium complexes, have a broad absorption band at around 500 nm to 600nm as the absorption band on the longest wavelength side (as a matter offact, the broad absorption band can be on a shorter or longer wavelengthside depending on emission wavelengths). This absorption band is mainlybased on a triplet MLCT (metal to ligand charge transfer) transition.Note that it is considered that the absorption band also includesabsorptions based on a triplet π-π* transition and a singlet MLCTtransition, and that these absorptions overlap one another to form abroad absorption band on the longest wavelength side in the absorptionspectrum. In other words, it can be considered that the differencebetween the lowest singlet excited state and the lowest triplet excitedstate is small, and absorptions based on these states overlap each otherto form a broad absorption band on the longest wavelength side in theabsorption spectrum. Therefore, as described above, it is preferablethat the broad absorption band on the longest wavelength side largelyoverlap with the emission spectrum of the host material when anorganometallic complex (especially iridium complex) is used as the guestmaterial.

Here, first, energy transfer from a host material in a triplet excitedstate will be considered. From the above-described discussion, it ispreferable that, in energy transfer from a triplet excited state, thephosphorescent spectrum of the host material and the absorption band onthe longest wavelength side of the guest material largely overlap eachother.

Note that a fluorescent compound is generally used as the host material;thus, phosphorescence lifetime (τ) is a millisecond or longer which isextremely long (i.e., k_(r)+k_(n) is low). This is because thetransition from the triplet excited state to the ground state (singlet)is a forbidden transition. Formula (3) shows that this is favorable toenergy transfer efficiency Φ_(ET). This also suggests that energy isgenerally likely to be transferred from the host material in the tripletexcited state to the guest material in the triplet excited state.

However, a question here is energy transfer from the host material inthe singlet excited state. In order to efficiently perform not onlyenergy transfer from the triplet excited state but also energy transferfrom the singlet excited state, it is clear from the above-describeddiscussion that the host material needs to be designed so as to have notonly its phosphorescent spectrum but also its fluorescent spectrumoverlapping with the absorption band on the longest wavelength side ofthe guest material. In other words, unless the host material is designedso as to have its fluorescent spectrum in a position similar to that ofits phosphorescent spectrum, it is not possible to achieve efficientenergy transfer from the host material in both the singlet excited stateand the triplet excited state.

However, the S₁ level generally differs greatly from the T₁ level (S₁level>T₁ level); therefore, the fluorescence emission wavelength alsodiffers greatly from the phosphorescence emission wavelength(fluorescence emission wavelength<phosphorescence emission wavelength).For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which iscommonly used as a host material in a light-emitting element including aphosphorescent compound, has a phosphorescent spectrum at around 500 nmand has a fluorescent spectrum at around 400 nm, which are largelydifferent by about 100 nm. This example also shows that it is extremelydifficult to design a host material so as to have its fluorescentspectrum in a position similar to that of its phosphorescent spectrum.Therefore, the present inventors consider it a major challenge toimprove efficiency in energy transfer from the host material in thesinglet excited state to the guest material.

Note that fluorescence lifetime (τ) of a fluorescent compound that isused as the host material is on the order of nanoseconds which isextremely short (i.e., k_(r)+k_(n) is high). This is because thetransition from the singlet excited state to the ground state (singlet)is an allowed transition. Formula (3) shows that this is unfavorable toenergy transfer efficiency Φ_(ET). This also suggests that energy isgenerally unlikely to be transferred from the host material in thesinglet excited state to the guest material.

One embodiment of the present invention is a useful technique which canovercome such a problem of the efficiency of the energy transfer fromthe host material in the singlet excited state to the guest material.

Note that it has been considered so far that a light-emitting elementincluding a phosphorescent compound can theoretically achieve aninternal quantum efficiency of 100% because intersystem crossing makesit possible to convert both the single excited state and the tripletexcited state into light emission (refer to “(1) direct recombinationprocess” described above). In addition, it has been discussed that alight-emitting element having an external quantum efficiency as high as20% under the assumption that the light extraction efficiency is 20%achieved an internal quantum efficiency of substantially 100%. However,it is considered in fact that these conventional light-emitting elementshave not achieved an internal quantum efficiency of 100% because theabove-described energy transfer from the singlet excited state of thehost material has been overlooked. This is based on the fact that thepresent inventors have achieved an external quantum efficiency of 27% ormore by carrying out one embodiment of the present invention which isdescribed below (see FIG. 12 and Example 2). It can be said that thevalue is equal to or exceeds a conventional theoretical limit ofexternal quantum efficiency. In other words, an external quantumefficiency of at least 27% or more corresponds to an internal quantumefficiency of 100%, and one embodiment of the present invention is auseful technique for achieving it. Note that this indicates that aconventional external quantum efficiency of 20% can be estimated tocorrespond to an internal quantum efficiency of 75% or less.

As described above, by application of one embodiment of the presentinvention, a light-emitting element with high external quantumefficiency can be provided.

One Embodiment of Present Invention

One embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex, inwhich an emission spectrum of the exciplex overlaps with an absorptionband located on the longest wavelength side of an absorption spectrum ofthe phosphorescent compound, and in which a peak wavelength of theemission spectrum of the exciplex is longer than or equal to a peakwavelength of the absorption band located on the longest wavelength sideof the absorption spectrum of the phosphorescent compound.

The first organic compound and the second organic compound form anexciplex (also referred to as excited complex) through recombination ofcarriers (i.e., electrons and holes) (or from a singlet exciton). In thecase where the exciplex formed emits light, the emission wavelengththereof is located on the longer wavelength side as compared to theemission wavelength (fluorescent wavelength) of each of the first andsecond organic compounds. In other words, by formation of the exciplex,the fluorescent spectrum of the first organic compound and thefluorescent spectrum of the second organic compound can be convertedinto an emission spectrum which is located on the longer wavelengthside.

Therefore, as illustrated in FIG. 5, even when the fluorescent spectrumof the first organic compound (or the second organic compound) islocated on the shorter wavelength side as compared to the absorptionband of the phosphorescent compound which is located on the longestwavelength side, and does not have an overlap with the absorption band,an emission spectrum with a long wavelength can be obtained by formingan exciplex so as to have a large overlap with the absorption band. Thelight-emitting element of one embodiment of the present inventiontransfers energy by utilizing the overlap between the emission spectrumof the exciplex and the absorption spectrum of the phosphorescentcompound and thus has high energy transfer efficiency. Therefore, in oneembodiment of the present invention, a light-emitting element havinghigh external quantum efficiency can be obtained.

In addition, the exciplex exists only in an excited state and thus hasno ground state capable of absorbing energy. Therefore, a phenomenon inwhich the phosphorescent compound is deactivated by reverse energytransfer of the singlet excited state and triplet excited state of thephosphorescent compound to the exciplex before light emission (i.e.,emission efficiency is lowered) is not considered to occur in principle.This also contributes to improvement of external quantum efficiency.

In addition, the exciplex is considered to have an extremely smalldifference between singlet excited energy and triplet excited energy. Inother words, the emission spectrum of the exciplex from the singletstate and the emission spectrum thereof from the triplet state arehighly close to each other. Accordingly, in the case where a design isimplemented such that the emission spectrum of the exciplex (generallythe emission spectrum of the exciplex from the singlet state) overlapswith the absorption band of the phosphorescent compound on the longestwavelength side as described above, the emission spectrum of theexciplex from the triplet state (which is not observed at roomtemperature and not observed even at low temperature in many cases) alsooverlaps with the absorption band of the phosphorescent compound whichis located on the longest wavelength side. More specifically, this meansthat energy can be efficiently transferred to the phosphorescentcompound from the exciplex in both the singlet state and the tripletstate.

Molecular orbital calculations were performed as described below toverify whether or not an exciplex actually has such characteristics. Ingeneral, a combination of a heteroaromatic compound and an aromaticamine often forms an exciplex under the influence of the lowestunoccupied molecular orbital (LUMO) level of the heteroaromatic compoundwhich is deeper than the LUMO level of the aromatic amine (the propertyof easily accepting electrons) and the highest occupied molecularorbital (HOMO) level of the aromatic amine which is shallower than theHOMO level of the heteroaromatic compound (the property of easilyaccepting holes). Thus, calculations were performed using a combinationof dibenzo[f,h]quinoxaline (abbreviation: DBq) which is a typicalskeleton forming the LUMO of a heteroaromatic compound andtriphenylamine (abbreviation: TPA) which is a typical skeleton formingthe HOMO of an aromatic amine.

First, the optimal molecular structures and the excitation energies ofDBq alone and TPA alone in the lowest singlet excited state (S₁) and thelowest triplet excited state (T₁) were calculated using thetime-dependent density functional theory (TD-DFT). Furthermore, theexcitation energy of a dimer of DBq and TPA was also calculated. In theDFT, the total energy is represented as the sum of potential energy,electrostatic energy between electrons, electronic kinetic energy, andexchange-correlation energy including all the complicated interactionsbetween electrons. Also in the DFT, an exchange-correlation interactionis approximated by a functional (a function of another function) of oneelectron potential represented in terms of electron density to enablehigh-speed, high-accuracy calculations. Here, B3LYP which was a hybridfunctional was used to specify the weight of each parameter related toexchange-correlation energy. In addition, as a basis function, 6-311 (abasis function of a triple-split valence basis set using threecontraction functions for each valence orbital) was applied to all theatoms. By the above basis function, for example, 1 s to 3 s orbitals areconsidered in the case of hydrogen atoms, while 1 s to 4 s and 2 p to 4p orbitals are considered in the case of carbon atoms. Furthermore, toimprove calculation accuracy, the p function and the d function aspolarization basis sets were added to hydrogen atoms and atoms otherthan hydrogen atoms, respectively.

Note that Gaussian 09 was used as a quantum chemistry computationalprogram. A high performance computer (Altix 4700, manufactured by SGIJapan, Ltd.) was used for the calculations.

First, the HOMO levels and the LUMO levels of DBq alone, TPA alone, anda dimer of DBq and TPA were calculated. FIG. 17 shows the HOMO levelsand the LUMO levels, and FIGS. 18A1, 18A2, 18B1, 18B2, 18C1, and 18C2show HOMO and LUMO distributions.

FIG. 18A1 shows the LUMO distribution of DBq alone; FIG. 18A2, the HOMOdistribution of DBq alone; FIG. 18B1, the LUMO distribution of TPAalone; FIG. 18B2, the HOMO distribution of TPA alone; FIG. 18C1, theLUMO distribution of the dimer of DBq and TPA; and FIG. 18C2, the HOMOdistribution of the dimer of DBq and TPA.

As shown in FIG. 17, it is suggested that the dimer of DBq and TPA formsan exciplex of DBq and TPA under the influence of the LUMO level (−1.99eV) of DBq which is deeper (lower) than the LUMO level of TPA and theHOMO level (−5.21 eV) of TPA which is shallower (higher) than the HOMOlevel of DBq. In fact, as is clear from FIGS. 18C1 and 18C2, the LUMO ofthe dimer of DBq and TPA is distributed on the DBq side, and the HOMOthereof is distributed on the TPA side.

Next, excitation energies obtained from the optimal molecular structuresof DBq alone in S₁ and T₁ will be shown. Here, the S₁ and T₁ excitationenergies correspond to fluorescence and phosphorescence wavelengths,respectively, obtained from DBq alone. The S₁ excitation energy of DBqalone is 3.294 eV, and the fluorescence wavelength is 376.4 nm. The T₁excitation energy of DBq alone is 2.460 eV, and the phosphorescencewavelength is 504.1 nm.

In addition, excitation energies obtained from the optimal molecularstructures of TPA alone in S₁ and T₁ will be shown. Here, the S₁ and T₁excitation energies correspond to fluorescence and phosphorescencewavelengths, respectively, obtained from TPA alone. The S₁ excitationenergy of TPA alone is 3.508 eV, and the fluorescence wavelength is353.4 nm. The T₁ excitation energy of TPA alone is 2.610 eV, and thephosphorescence wavelength is 474.7 nm.

Furthermore, excitation energies obtained from the optimal molecularstructures of the dimer of DBq and TPA in S₁ and T₁ will be shown. TheS₁ and T₁ excitation energies correspond to fluorescence andphosphorescence wavelengths, respectively, obtained from the dimer ofDBq and TPA. The S₁ excitation energy of the dimer of DBq and TPA is2.036 eV, and the fluorescence wavelength is 609.1 nm. The T₁ excitationenergy of the dimer of DBq and TPA is 2.030 eV, and the phosphorescencewavelength is 610.0 nm.

It is found from the above that each of the phosphorescence wavelengthsof DBq alone and TPA alone is shifted to the longer wavelength side byabout 100 nm as compared to the fluorescence wavelength. This resultshows a tendency similar to that of CBP (measured values) describedabove and supports the validity of the calculations.

On the other hand, it is found that the fluorescence wavelength of thedimer of DBq and TPA is located on the longer wavelength side ascompared to the fluorescence wavelengths of DBq alone and TPA alone.This result shows a tendency similar to that of examples (measuredvalues) described below and supports the validity of the calculations.It is also found that the difference between the fluorescence wavelengthand the phosphorescence wavelength of the dimer of DBq and TPA is only0.9 nm and that these wavelengths are substantially the same.

These results indicate that the exciplex can integrate the singletexcitation energy and the triplet excitation energy into substantiallythe same energy. Therefore, it is indicated as described above that theexciplex can efficiently transfer energy to the phosphorescent compoundfrom both the singlet state and the triplet state thereof.

Such an effect is specific to the use of an exciplex as a medium forenergy transfer. In general, energy transfer from the singlet excitedstate or triplet excited state of a host material to a phosphorescentcompound is considered. On the other hand, one embodiment of the presentinvention greatly differs from a conventional technique in that anexciplex of a host material and another material (an exciplex of a firstorganic compound and a second organic compound) is formed first andenergy transfer from the exciplex is used. In addition, this differenceprovides unprecedentedly high emission efficiency.

Note that in general, the use of an exciplex for a light-emitting layerof a light-emitting element has a value such as being capable ofcontrolling the emission color, but usually causes a significantdecrease in emission efficiency. Therefore, the use of an exciplex hasbeen considered unsuitable for obtaining a highly efficientlight-emitting element. However, the present inventors have found thatthe use of an exciplex as a medium for energy transfer to aphosphorescent compound enables, on the contrary, emission efficiency tobe maximized as shown in one embodiment of the present invention. Thistechnical idea conflicts with the conventional fixed idea.

Further, in the light-emitting element of one embodiment of the presentinvention, the threshold value of voltage with which an exciplex isformed through carrier recombination (or from a singlet exciton) dependson the energy of a peak of the emission spectrum of the exciplex. Whenthe emission spectrum of the exciplex peaks at 620 nm (2.0 eV), forexample, the threshold value of voltage that is needed when the exciplexis formed with electric energy is also approximately 2.0 V.

Here, when the energy of the peak of the emission spectrum of theexciplex is too high (i.e., when the wavelength is too short), thethreshold value of the voltage with which an exciplex is formed alsoincreases. That case is not preferred because higher voltage is neededto make the phosphorescent compound emit light by energy transfer fromthe exciplex to the phosphorescent compound, and thus extra energy isconsumed.

In view of this, it is preferable that energy of the peak of theemission spectrum of the exciplex be lower (the wavelength be longer)because in that case, the threshold value of the voltage is smaller.Accordingly, the light-emitting element of one embodiment of the presentinvention, in which the peak wavelength of the emission spectrum of theexciplex is longer than or equal to the peak wavelength of theabsorption band located on the longest wavelength side of the absorptionspectrum of the phosphorescent compound, can be driven at low drivevoltage. In addition, in the light-emitting element of one embodiment ofthe present invention, even when the peak wavelength of the emissionspectrum of the exciplex is longer than or equal to the peak wavelengthof the absorption spectrum of the phosphorescent compound, energy can betransferred utilizing the overlap between the emission spectrum of theexciplex and the absorption band located on the longest wavelength sideof the absorption spectrum of the phosphorescent compound, which leadsto high emission efficiency of the light-emitting element. As describedabove, high emission efficiency (external quantum efficiency) isobtained with the drive voltage reduced, whereby high power efficiencycan be achieved.

In the above-described light-emitting element, since the peak wavelengthof the emission spectrum of the exciplex is particularly long, the drivevoltage can be lower. This can be explained as follows.

One embodiment of the present invention includes a light-emittingelement in which the peak wavelength of the emission spectrum of theexciplex is longer than or equal to the peak wavelength of theabsorption band located on the longest wavelength side of the absorptionspectrum of the phosphorescent compound (i.e., the energy of theemission peak of the exciplex is lower than or equal to the energy ofthe absorption peak of the phosphorescent compound). Therefore, in thelight-emitting element, a value of voltage with which an exciplex isformed through carrier recombination is smaller than a value of voltagewith which the phosphorescent compound starts to emit light by carrierrecombination.

In other words, even when voltage that has a value smaller than that ofvoltage with which the phosphorescent compound starts to emit light isapplied to the light-emitting element, carrier recombination occurs andan exciplex is formed; thus, recombination current starts to flow in thelight-emitting element. Therefore, a light-emitting element with lowerdrive voltage (with more favorable voltage-current characteristics) canbe provided.

Accordingly, at the time when the voltage reaches a value with which thephosphorescent compound starts to emit light, a sufficient number ofcarriers exist in the light-emitting layer and carrier recombinationwhich can contribute to light emission of the phosphorescent compoundsmoothly occurs many times. Therefore, luminance becomes remarkably highat a voltage close to the threshold voltage (emission start voltage) ofthe phosphorescent compound. In other words, a curve representing thevoltage-luminance characteristics can be steep in a rising portion nearthe emission start voltage; thus, drive voltage needed to obtain desiredluminance can be low. Further, to obtain practical luminance, driving isperformed with voltage higher than or equal to the threshold voltage(emission start voltage) of the phosphorescent compound, in which caseemitted light originates mostly from the phosphorescent compound and thelight-emitting element is thus allowed to have high current efficiency.

Note that the phosphorescent compound used in one embodiment of thepresent invention has a singlet absorption spectrum and a tripletabsorption spectrum which are located close to each other. Moreover, inan exciplex formed in one embodiment of the present invention, a peak ofan emission spectrum of the exciplex from the singlet state and a peakof an emission spectrum of the exciplex from the triplet state can beconsidered to be located close to each other. Therefore, even when thepeak of the emission spectrum of the exciplex (usually the emissionspectrum of the exciplex from the singlet state) is located close to thepeak of the emission spectrum of the phosphorescent compound, quenchingof triplet excitation energy of the phosphorescent compound due to theexciplex in the triplet state can be suppressed. In the first place, theexciplex does not have an absorption spectrum; thus, a phenomenon isless likely to occur in which the triplet excitation energy of thephosphorescent compound transfers to the exciplex to be quenched. Thisalso suggests that the light-emitting element of one embodiment of thepresent invention has high external quantum efficiency. The above alsois an advantage of using an exciplex.

One embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex, inwhich an emission spectrum of the exciplex overlaps with an absorptionband located on the longest wavelength side of an absorption spectrum ofthe phosphorescent compound, and in which a difference between a peakwavelength of the emission spectrum of the exciplex and a peakwavelength of an emission spectrum of the phosphorescent compound is 30nm or less.

The light-emitting element can be driven at low drive voltage and havesufficiently high emission efficiency when the peak of the emissionspectrum of the exciplex (usually the emission spectrum of the exciplexfrom the singlet state) is located close to the peak of the emissionspectrum of the phosphorescent compound as described above. The effectof a reduction in drive voltage is enhanced especially when the peak ofthe emission spectrum of the exciplex is located in a region rangingfrom the peak of the emission spectrum of the phosphorescent compound toa wavelength 30 nm longer than the peak of the emission spectrum of thephosphorescent compound. Further, relatively high emission efficiencycan be maintained when the peak of the emission spectrum of the exciplexis in a region ranging from the peak of the emission spectrum of thephosphorescent compound to a wavelength 30 nm shorter than the peak ofthe emission spectrum of the phosphorescent compound.

However, when the peak of the emission spectrum of the exciplex islocated on the longer wavelength side as compared to the peak of theemission spectrum of the phosphorescent compound, the external quantumefficiency of the light-emitting element is reduced in some cases. Thereason for this is that under this condition, the overlap between theemission spectrum of the exciplex and the absorption band located on thelongest wavelength side of the absorption spectrum of the phosphorescentcompound is decreased, excitation energy transfer from the exciplex tothe phosphorescent compound is thus unlikely to occur, and the exciplexitself is easily deactivated by emitting the excitation energy as lightor heat.

Thus, for achievement of extremely high emission efficiency, anotherembodiment of the present invention is a light-emitting element whichincludes a light-emitting layer containing a phosphorescent compound, afirst organic compound, and a second organic compound between a pair ofelectrodes, in which a combination of the first organic compound and thesecond organic compound forms an exciplex, in which an emission spectrumof the exciplex overlaps with an absorption band located on the longestwavelength side of an absorption spectrum of the phosphorescentcompound, and in which a peak wavelength of the emission spectrum of theexciplex is longer than or equal to a peak wavelength of the absorptionband located on the longest wavelength side of the absorption spectrumof the phosphorescent compound and shorter than or equal to a peakwavelength of an emission spectrum of the phosphorescent compound.

In the light-emitting element, the peak wavelength of the emissionspectrum of the exciplex is shorter than or equal to the peak wavelengthof the emission spectrum of the phosphorescent compound, and thus theoverlap between the emission spectrum of the exciplex and the absorptionband located on the longest wavelength side of the absorption spectrumof the phosphorescent compound is increased. Therefore, excitationenergy can be efficiently transferred from the exciplex to thephosphorescent compound. Accordingly, energy deactivation can besuppressed. Accordingly, a light-emitting element having low drivevoltage and high external quantum efficiency can be obtained.

Specifically, it is preferable that a difference between the peakwavelength of the emission spectrum of the exciplex and the peakwavelength of the emission spectrum of the phosphorescent compound be 30nm or less.

In addition, in one embodiment of the present invention, an exciplex isformed from a singlet exciton of the first organic compound or thesecond organic compound.

In a light-emitting element of one embodiment of the present invention,a possible elementary process of formation of an exciplex is that one ofthe first and second organic compounds forms a singlet exciton and theninteracts with the other in the ground state. As described above, theemission spectrum of the exciplex and the absorption spectrum of thephosphorescent compound can largely overlap; thus, energy transferefficiency can be increased. Accordingly, a light-emitting elementhaving high external quantum efficiency can be obtained.

The singlet exciton has a short excitation lifetime (small t) asdescribed above. Thus, there is a problem in that part of excitationenergy is deactivated (through light emission or thermal deactivation)before the excitation energy is transferred from the singlet exciton toa guest material Φ_(ET) tends to be small in Formula (3)). However, inone embodiment of the present invention, such deactivation of excitationenergy can be suppressed because the singlet exciton rapidly forms anexciplex. Furthermore, the exciplex has a relatively long excitationlifetime, which is considered favorable to energy transfer efficiencyΦ_(ET). Accordingly, the deactivation of the singlet excitation energyof the host material that may affect not only the efficiency of anelement but also the lifetime thereof can be suppressed by applicationof one embodiment of the present invention, so that a light-emittingelement having a long lifetime can be obtained.

In one embodiment of the present invention, it is also preferable thatthe excitation energy of the exciplex be sufficiently transferred to thephosphorescent compound, and that light emission from the exciplex benot substantially observed. Therefore, energy is preferably transferredto the phosphorescent compound through the exciplex so that thephosphorescent compound emits phosphorescence.

According to the above-described concept of energy transfer, oneembodiment of the present invention is effective in the case where atleast one of the first and second organic compounds is a fluorescentcompound (i.e., a compound which is likely to undergo light emission orthermal deactivation from the singlet excited state). Therefore, it ispreferable that at least one of the first and second organic compoundsbe a fluorescent compound.

Note that in the case where a phosphorescent compound is used as anorganic compound serving as a host material, the organic compound itselfis likely to emit light and unlikely to allow energy to be transferredto a guest material. In this case, it is favorable if the organiccompound could emit light efficiently, but it is difficult to achievehigh emission efficiency because the organic compound serving as a hostmaterial causes the problem of concentration quenching. For this reason,it is preferable that the organic compound be a fluorescent compound andenergy transfer be achieved with the above-described composition.

In addition, in one embodiment of the present invention, it ispreferable that the phosphorescent compound be an organometalliccomplex.

The exciplex used in one embodiment of the present invention will bedescribed in detail below.

<Exciplex>

The exciplex (excited complex) is formed by an interaction betweendissimilar molecules in excited states. The exciplex is generally knownto be easily formed between a material having a relatively deep LUMOlevel and a material having a relatively shallow HOMO level.

An emission wavelength depends on a difference in energy between theHOMO level and the LUMO level. When the energy difference is large, theemission wavelength is short. When the energy difference is small, theemission wavelength is long.

Here, the HOMO levels and LUMO levels of the first organic compound andthe second organic compound used in one embodiment of the presentinvention are different from each other. Specifically, the energy levelsare higher in the following order: the HOMO level of the first organiccompound<the HOMO level of the second organic compound<the LUMO level ofthe first organic compound<the LUMO level of the second organic compound(see FIG. 6).

When the exciplex is formed by these two organic compounds, the LUMOlevel and the HOMO level of the exciplex originate from the firstorganic compound and the second organic compound, respectively (see FIG.6). Therefore, the energy difference of the exciplex is smaller than theenergy difference of the first organic compound and the energydifference of the second organic compound. In other words, the emissionwavelength of the exciplex is longer than the emission wavelengths ofthe first organic compound and the second organic compound.

The formation process of the exciplex used in one embodiment of thepresent invention is considered to be roughly classified into twoprocesses.

<<Electroplex>>

In this specification, the term “electroplex” means that the firstorganic compound in the ground state and the second organic compound inthe ground state directly form an exciplex.

As described above, in general, when an electron and a hole arerecombined in a host material, excitation energy is transferred from thehost material in an excited state to a guest material, whereby the guestmaterial is brought into an excited state to emit light.

At this time, before the excitation energy is transferred from the hostmaterial to the guest material, the host material itself emits light orthe excitation energy turns into thermal energy, which leads to partialdeactivation of the excitation energy. In particular, when the hostmaterial is in a singlet excited state, excitation lifetime is shorterthan that when it is in a triplet excited state, which easily leads todeactivation of singlet excitation energy. The deactivation ofexcitation energy is one of causes for a decrease in lifetime of alight-emitting element.

However, in one embodiment of the present invention, an electroplex isformed from the first organic compound and the second organic compoundhaving carriers (cation or anion); therefore, formation of a singletexciton having a short excitation lifetime can be suppressed. In otherwords, there can be a process where an exciplex is directly formedwithout formation of a singlet exciton. Thus, deactivation of thesinglet excitation energy can be inhibited. Accordingly, alight-emitting element having a long lifetime can be obtained.

For example, in the case where the first organic compound is a compoundhaving an electron-trapping property and the second organic compound isa compound having a hole-trapping property, an electroplex is formeddirectly from an anion of the first organic compound and a cation of thesecond organic compound. It is a novel concept to obtain alight-emitting element having high emission efficiency by suppressingthe generation of the singlet excited state of a host material andtransferring energy from an electroplex to a guest material, in theabove-described manner. Note that the generation of the triplet excitedstate of the host material is similarly suppressed and an electroplex isdirectly formed; therefore, energy transfer is considered to occur fromthe electroplex to the guest material. This mechanism is also novel.

The emission spectrum of the electroplex formed is located on the longerwavelength side as compared to the emission wavelength of each of thefirst and second organic compounds.

The overlap between the emission spectrum of the electroplex and theabsorption spectrum of the phosphorescent compound is larger than theoverlap between the emission spectrum of the first organic compound (orthe second organic compound) and the absorption spectrum of thephosphorescent compound. The light-emitting element of one embodiment ofthe present invention transfers energy by utilizing the overlap betweenthe emission spectrum of the electroplex and the absorption spectrum ofthe phosphorescent compound and thus has high energy transferefficiency. Therefore, in one embodiment of the present invention, alight-emitting element having high external quantum efficiency can beobtained.

<<Formation of Exciplex by Exciton>>

As another process, there is thought to be an elementary process whereone of the first and second organic compounds forms a singlet excitonand then interacts with the other in the ground state to form anexciplex. Unlike an electroplex, a singlet excited state of the firstorganic compound or the second organic compound is temporarily generatedin this case, but this is rapidly converted into an exciplex, and thus,deactivation of single excitation energy can be inhibited. Thus, it ispossible to inhibit deactivation of excitation energy of the firstorganic compound or the second organic compound. Accordingly, in oneembodiment of the present invention, a light-emitting element having along lifetime can be obtained. Note that it is considered that thetriplet excited state of the host material is also rapidly convertedinto an exciplex and energy is transferred from the exciplex to theguest material.

The emission spectrum of the exciplex formed is located on the longerwavelength side as compared to the emission wavelength of each of thefirst and second organic compounds.

The overlap between the emission spectrum of the exciplex and theabsorption spectrum of the phosphorescent compound is larger than theoverlap between the emission spectrum of the first organic compound (orthe second organic compound) and the absorption spectrum of thephosphorescent compound. The light-emitting element of one embodiment ofthe present invention transfers energy by utilizing the overlap betweenthe emission spectrum of the exciplex and the absorption spectrum of thephosphorescent compound and thus has high energy transfer efficiency.Accordingly, in one embodiment of the present invention, alight-emitting element having high external quantum efficiency can beobtained.

For example, in the case where the first organic compound is a compoundhaving an electron-trapping property, the second organic compound is acompound having a hole-trapping property, and the difference between theHOMO levels and the difference between the LUMO levels of thesecompounds are large (specifically, 0.3 eV or more), electrons areselectively injected into the first organic compound and holes areselectively injected into the second organic compound. In this case, itis thought that the process where an electroplex is formed takesprecedence over the process where an exciplex is formed through asinglet exciton.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 7A to 7C.

FIG. 7A illustrates a light-emitting element including an EL layer 102between a first electrode 103 and a second electrode 108. Thelight-emitting element in FIG. 7A includes a hole-injection layer 701, ahole-transport layer 702, a light-emitting layer 703, anelectron-transport layer 704, and an electron-injection layer 705 whichare stacked over the first electrode 103 in this order, and the secondelectrode 108 provided thereover.

The first electrode 103 is preferably formed using any of metals,alloys, conductive compounds, mixtures thereof, and the like which havea high work function (specifically, 4.0 eV or more). Specific examplesinclude indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tinoxide containing silicon or silicon oxide, indium oxide-zinc oxide(indium zinc oxide), indium oxide containing tungsten oxide and zincoxide (IWZO), and the like. Films of these conductive metal oxides areusually formed by a sputtering method, but may be formed by applicationof a sol-gel method or the like. For example, an indium oxide-zinc oxidefilm can be formed by a sputtering method using a target in which zincoxide is added to indium oxide at 1 wt % to 20 wt %. Further, an IWZOfilm can be formed by a sputtering method using a target in whichtungsten oxide is added to indium oxide at 0.5 wt % to 5 wt % and zincoxide is added to indium oxide at 0.1 wt % to 1 wt %. Other examples aregraphene, gold, platinum, nickel, tungsten, chromium, molybdenum, iron,cobalt, copper, palladium, nitrides of metal materials (e.g., titaniumnitride), and the like.

Note that when a layer included in the EL layer 102 and formed incontact with the first electrode 103 is formed using a later-describedcomposite material formed by combining an organic compound and anelectron acceptor (an acceptor), as a substance used for the firstelectrode 103, any of a variety of metals, alloys, electricallyconductive compounds, mixtures thereof, and the like can be usedregardless of the work function; for example, aluminum, silver, an alloycontaining aluminum (e.g., Al—Si), or the like can also be used.

The first electrode 103 can be formed by, for example, a sputteringmethod, an evaporation method (including a vacuum evaporation method),or the like.

The second electrode 108 is preferably formed using any of metals,alloys, electrically conductive compounds, mixtures thereof, and thelike which have a low work function (preferably, 3.8 eV or lower).Specific examples thereof include elements that belong to Groups 1 and 2in the periodic table, that is, alkali metals such as lithium andcesium, alkaline earth metals such as calcium and strontium, magnesium,alloys thereof (e.g., Mg—Ag and Al—Li), rare-earth metals such aseuropium and ytterbium, alloys thereof, aluminum, silver, and the like.

When a layer included in the EL layer 102 and formed in contact with thesecond electrode 108 is formed using a later-described compositematerial formed by combining an organic compound and an electron donor(a donor), any of a variety of conductive materials, such as Al, Ag,ITO, and indium oxide-tin oxide containing silicon or silicon oxide, canbe used regardless of the work function.

Note that when the second electrode 108 is formed, a vacuum evaporationmethod or a sputtering method can be used. In the case of using a silverpaste or the like, a coating method, an inkjet method, or the like canbe used.

The EL layer 102 includes at least the light-emitting layer 703. Forpart of the EL layer 102, a known substance can be used, and either alow molecular compound or a high molecular compound can be used. Notethat substances forming the EL layer 102 may consist of organiccompounds or may include an inorganic compound as a part.

Further, as illustrated in FIG. 7A, the EL layer 102 includes not onlythe light-emitting layer 703 but also an appropriate combination of thefollowing layers: the hole-injection layer 701 including a substancehaving a high hole-injection property, the hole-transport layer 702including a substance having a high hole-transport property, theelectron-transport layer 704 including a substance having a highelectron-transport property, the electron-injection layer 705 includinga substance having a high electron-injection property, and the like.

The hole-injection layer 701 is a layer that contains a substance havinga high hole-injection property. As the substance having a highhole-injection property, a metal oxide such as molybdenum oxide,titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromiumoxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide,tungsten oxide, or manganese oxide can be used. Alternatively, aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper(II) phthalocyanine (abbreviation: CuPc) can be used.

Other examples of the substance which can be used are aromatic aminecompounds and the like which are low molecular organic compounds, suchas 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Still other examples of the substance which can be used are highmolecular compounds (e.g., oligomers, dendrimers, and polymers), such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine](abbreviation:Poly-TPD), and high molecular compounds to which acid is added, such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),and polyaniline/poly(styrenesulfonic acid) (PAni/PSS).

For the hole-injection layer 701, the composite material formed bycombining an organic compound and an electron acceptor (an acceptor) maybe used. Such a composite material, in which holes are generated in theorganic compound by the electron acceptor, has high hole-injection andhole-transport properties. In this case, the organic compound ispreferably a material excellent in transporting the generated holes (asubstance having a high hole-transport property).

Examples of the organic compound used for the composite material can bea variety of compounds, such as aromatic amine compounds, carbazolederivatives, aromatic hydrocarbons, and high molecular compounds (e.g.,oligomers, dendrimers, and polymers). The organic compound used for thecomposite material is preferably an organic compound having a highhole-transport property, and is specifically preferably a substancehaving a hole mobility of 10⁻⁶ cm²/Vs or more. Note that other thanthese substances, any substance that has a property of transporting moreholes than electrons may be used. Organic compounds that can be used forthe composite material will be specifically described below.

Examples of the organic compound that can be used for the compositematerial are aromatic amine compounds, such as TDATA, MTDATA, DPAB,DNTPD, DPA3B, PCzPCA1, PCzPCA2,PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation:NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP),and carbazole derivatives, such as 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene(abbreviation: CzPA),9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), and 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Other examples of the organic compound that can be used are aromatichydrocarbon compounds, such as2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene, and2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene.

Other examples of the organic compound that can be used are aromatichydrocarbon compounds, such as2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), and 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA).

Further, examples of the electron acceptor are organic compounds, suchas 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil, oxides of transition metals, oxides of metalsthat belong to Groups 4 to 8 in the periodic table, and the like.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable because of their high electron-accepting property.Among these, molybdenum oxide is especially preferable since it isstable in the air, has a low hygroscopic property, and is easy tohandle.

The composite material may be formed using the above-described electronacceptor and the above-described high molecular compound such as PVK,PVTPA, PTPDMA, or Poly-TPD, and may be used for the hole-injection layer701.

The hole-transport layer 702 is a layer that contains a substance havinga high hole-transport property. Examples of the substance having a highhole-transport property are aromatic amine compounds such as NPB, TPD,BPAFLP, 4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). The substances mentioned here are mainlysubstances that have a hole mobility of 10⁻⁶ cm²/Vs or more. Note thatother than these substances, any substance that has a property oftransporting more holes than electrons may be used. Note that the layercontaining a substance having a high hole-transport property is notlimited to a single layer, and may be a stack of two or more layerscontaining any of the above substances.

For the hole-transport layer 702, a carbazole derivative such as CBP,CzPA, or PCzPA or an anthracene derivative such as t-BuDNA, DNA, orDPAnth may be used.

For the hole-transport layer 702, a high molecular compound such as PVK,PVTPA, PTPDMA, or Poly-TPD can also be used.

The light-emitting layer 703 is a layer that contains a light-emittingsubstance. The light-emitting layer 703 of this embodiment contains aphosphorescent compound, a first organic compound, and a second organiccompound. The phosphorescent compound is a light-emitting substance(guest material). One of the first and second organic compounds, thecontent of which is higher than that of the other in the light-emittinglayer 703, is a host material. Embodiment 1 can be referred to forspecifics.

As the phosphorescent compound, an organometallic complex is preferable,and in particular, an iridium complex is preferable. In consideration ofenergy transfer due to Förster mechanism described above, the molarabsorption coefficient of the absorption band of the phosphorescentcompound which is located on the longest wavelength side is preferably2000 M⁻¹·cm⁻¹ or more, more preferably 5000 M⁻¹·cm⁻¹ or more. Examplesof the compound having such a high molar absorption coefficient arebis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(dpm)]),(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]), and the like.

For the first organic compound and the second organic compound, acombination of a compound which is likely to accept electrons (acompound having an electron-trapping property) and a compound which islikely to accept holes (a compound having a hole-trapping property) ispreferably employed. With such a composition, it is possible to obtainthe effect of improvement of emission efficiency and lifetime not onlyby energy transfer from an exciplex but also by adjustment of carrierbalance between hole transport and electron transport in alight-emitting layer.

As a typical example of a compound which is likely to accept electrons,heteroaromatic compounds can be given. For example, the following can begiven: 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTPDBq-II),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), and6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:6mDBTPDBq-II).

As typical examples of a compound which is likely to accept holes,aromatic amine compounds and carbazole compounds can be given. Forexample, the following can be given:4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA or 1-TNATA),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF),N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N-(9,9-dimethyl-2-N,N-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),N,N-bis[4-(carbazol-9-yl)phenyl]-N,N-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N-phenyl-N-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),4,4′-bis(N-{4-[N-(3-methylphenyl)-N-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2), and3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2).

Note that as for the first organic compound and the second organiccompound, the present invention is not limited to the above examples.The combination is determined so that an exciplex can be formed, theemission spectrum of the exciplex overlaps with the absorption spectrumof the phosphorescent compound, and the peak of the emission spectrum ofthe exciplex has a longer wavelength than the peak of the absorptionspectrum of the phosphorescent compound.

Note that in the case where a compound which is likely to acceptelectrons and a compound which is likely to accept holes are used forthe first organic compound and the second organic compound, carrierbalance can be controlled by the mixture ratio of the compounds. Thatis, another feature of one embodiment of the present invention is thatthe optimal carrier balance with which the probability of recombinationof holes and electrons in the light-emitting layer and the emissionefficiency are increased can be designed by setting the mixture ratio.In view of the carrier balance and formation of an exciplex, it ispreferable that the amount of the first organic compound and that of thesecond organic compound be not significantly different. Specifically,the ratio of the first organic compound to the second organic compoundis preferably 1:9 to 9:1.

Further, the exciplex may be formed at the interface between two layers.For example, when a layer containing the second organic compound and alayer containing the first organic compound are stacked, the exciplex isformed in the vicinity of the interface thereof. These two layers may beused as the light-emitting layer in one embodiment of the presentinvention. In that case, the phosphorescent compound is added to thevicinity of the interface. The phosphorescent compound may be added toone of the two layers or both.

The electron-transport layer 704 is a layer that contains a substancehaving a high electron-transport property. Examples of the substancehaving a high electron-transport property are metal complexes such asAlq₃, tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), BAlq,Zn(BOX)₂, and bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂). Other examples thereof are heteroaromatic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP),4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Stillother examples are high molecular compounds such aspoly(2,5-pyridine-diyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation:PF-Py), andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation:PF-BPy). The substances mentioned here are mainly substances having anelectron mobility of 10⁻⁶ cm²/Vs or more. Note that other than thesesubstances, any substance that has a property of transporting more holesthan electrons may be used for the electron-transport layer.

Further, the electron-transport layer is not limited to a single layer,and may be a stack of two or more layers containing any of the abovesubstances.

The electron-injection layer 705 is a layer that contains a substancehaving a high electron-injection property. Examples of the substancethat can be used for the electron-injection layer 705 are alkali metals,alkaline earth metals, and compounds thereof, such as lithium, cesium,calcium, lithium fluoride, cesium fluoride, calcium fluoride, andlithium oxide, rare earth metal compounds, such as erbium fluoride, andthe above-mentioned substances used for the electron-transport layer704.

Alternatively, a composite material formed by combining an organiccompound and an electron donor (a donor) may be used for theelectron-injection layer 705. Such a composite material, in whichelectrons are generated in the organic compound by the electron donor,has high electron-injection and electron-transport properties. Theorganic compound here is preferably a material excellent in transportingthe generated electrons, and specifically any of the above substances(such as metal complexes and heteroaromatic compounds) for theelectron-transport layer 704 can be used. As the electron donor, asubstance having an electron-donating property to the organic compoundmay be used. Preferable specific examples of the electron donor arealkali metals, alkaline earth metals, and rare earth metals, such aslithium, cesium, magnesium, calcium, erbium, and ytterbium. Any ofalkali metal oxides and alkaline earth metal oxides is preferable,examples of which are lithium oxide, calcium oxide, barium oxide, andthe like, and a Lewis base such as magnesium oxide or an organiccompound such as tetrathiafulvalene (abbreviation: TTF) can be used.

Note that the hole-injection layer 701, the hole-transport layer 702,the light-emitting layer 703, the electron-transport layer 704, and theelectron-injection layer 705 which are mentioned above can each beformed by a method such as an evaporation method (including a vacuumevaporation method), an inkjet method, or a coating method.

A plurality of EL layers may be stacked between the first electrode 103and the second electrode 108 as illustrated in FIG. 7B. In that case, acharge-generation layer 803 is preferably provided between a first ELlayer 800 and a second EL layer 801 which are stacked. Thecharge-generation layer 803 can be formed using the above-describedcomposite material. Further, the charge-generation layer 803 may have astacked structure including a layer containing the composite materialand a layer containing another material. In that case, as the layercontaining another material, a layer containing an electron-donatingsubstance and a substance with a high electron-transport property, alayer formed of a transparent conductive film, or the like can be used.As for a light-emitting element having such a structure, problems suchas energy transfer and quenching hardly occur, and a light-emittingelement which has both high emission efficiency and a long lifetime canbe easily obtained owing to a wider choice of materials. Moreover, alight-emitting element which provides phosphorescence from one of the ELlayers and fluorescence from the other of the EL layers can be readilyobtained. This structure can be combined with any of the above-describedstructures of the EL layer.

Furthermore, by making emission colors of EL layers different, light ofa desired color can be obtained from the light-emitting element as awhole. For example, the emission colors of first and second EL layersare complementary in a light-emitting element having the two EL layers,so that the light-emitting element can be made to emit white light as awhole. Further, the same applies to a light-emitting element havingthree or more EL layers.

As illustrated in FIG. 7C, the EL layer 102 may include thehole-injection layer 701, the hole-transport layer 702, thelight-emitting layer 703, the electron-transport layer 704, anelectron-injection buffer layer 706, an electron-relay layer 707, and acomposite material layer 708 which is in contact with the secondelectrode 108, between the first electrode 103 and the second electrode108.

It is preferable to provide the composite material layer 708 which is incontact with the second electrode 108, in which case damage caused tothe EL layer 102 particularly when the second electrode 108 is formed bya sputtering method can be reduced. The composite material layer 708 canbe formed using the above-described composite material in which anorganic compound having a high hole-transport property contains anacceptor substance.

Further, by providing the electron-injection buffer layer 706, aninjection barrier between the composite material layer 708 and theelectron-transport layer 704 can be reduced; thus, electrons generatedin the composite material layer 708 can be easily injected to theelectron-transport layer 704.

For the electron-injection buffer layer 706, a substance having a highelectron-injection property, such as an alkali metal, an alkaline earthmetal, a rare earth metal, a compound of the above metal (e.g., analkali metal compound (including an oxide such as lithium oxide, ahalide, and a carbonate such as lithium carbonate or cesium carbonate),an alkaline earth metal compound (including an oxide, a halide, and acarbonate), or a rare earth metal compound (including an oxide, ahalide, and a carbonate), can be used.

Further, in the case where the electron-injection buffer layer 706contains a substance having a high electron-transport property and adonor substance, the donor substance is preferably added so that themass ratio of the donor substance to the substance having a highelectron-transport property is in the range from 0.001:1 to 0.1:1. Notethat as the donor substance, an organic compound such astetrathianaphthacene (abbreviation: TTN), nickelocene, ordecamethylnickelocene can be used as well as an alkali metal, analkaline earth metal, a rare earth metal, and a compound of the abovemetal (e.g., an alkali metal compound (including an oxide such aslithium oxide, a halide, and a carbonate such as lithium carbonate orcesium carbonate), an alkaline earth metal compound (including an oxide,a halide, and a carbonate), and a rare earth metal compound (includingan oxide, a halide, and a carbonate)). Note that as the substance havinga high electron-transport property, a material similar to the materialfor the electron-transport layer 704 described above can be used.

Furthermore, it is preferable that the electron-relay layer 707 beformed between the electron-injection buffer layer 706 and the compositematerial layer 708. The electron-relay layer 707 is not necessarilyprovided; however, by providing the electron-relay layer 707 having ahigh electron-transport property, electrons can be rapidly transportedto the electron-injection buffer layer 706.

The structure in which the electron-relay layer 707 is sandwichedbetween the composite material layer 708 and the electron-injectionbuffer layer 706 is a structure in which the acceptor substancecontained in the composite material layer 708 and the donor substancecontained in the electron-injection buffer layer 706 are less likely tointeract with each other, and thus their functions hardly interfere witheach other. Therefore, an increase in drive voltage can be prevented.

The electron-relay layer 707 contains a substance having a highelectron-transport property and is formed so that the LUMO level of thesubstance having a high electron-transport property is located betweenthe LUMO level of the acceptor substance contained in the compositematerial layer 708 and the LUMO level of the substance having a highelectron-transport property contained in the electron-transport layer704. In the case where the electron-relay layer 707 contains a donorsubstance, the donor level of the donor substance is also controlled soas to be located between the LUMO level of the acceptor substancecontained in the composite material layer 708 and the LUMO level of thesubstance having a high electron-transport property contained in theelectron-transport layer 704. As a specific value of the energy level,the LUMO level of the substance having a high electron-transportproperty contained in the electron-relay layer 707 is preferably higherthan or equal to −5.0 eV, more preferably higher than or equal to −5.0eV and lower than or equal to −3.0 eV.

As the substance having a high electron-transport property contained inthe electron-relay layer 707, a phthalocyanine-based material or a metalcomplex having a metal-oxygen bond and an aromatic ligand is preferablyused.

As the phthalocyanine-based material contained in the electron-relaylayer 707, specifically, any of CuPc, a phthalocyanine tin(II) complex(SnPc), a phthalocyanine zinc complex (ZnPc), cobalt(II) phthalocyanine,β-form (CoPc), phthalocyanine iron (FePc), and vanadyl2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (PhO-VOPc), is preferablyused.

As the metal complex having a metal-oxygen bond and an aromatic ligand,which is contained in the electron-relay layer 707, a metal complexhaving a metal-oxygen double bond is preferably used. The metal-oxygendouble bond has an acceptor property (a property of easily acceptingelectrons); thus, electrons can be transferred (donated and accepted)more easily. Further, the metal complex having a metal-oxygen doublebond is considered stable. Thus, the use of the metal complex having themetal-oxygen double bond enables the light-emitting element to be drivenmore stably at low voltage.

As the metal complex having a metal-oxygen bond and an aromatic ligand,a phthalocyanine-based material is preferable. Specifically, any ofvanadyl phthalocyanine (VOPc), a phthalocyanine tin(IV) oxide complex(SnOPc), and a phthalocyanine titanium oxide complex (TiOPc) ispreferable because a metal-oxygen double bond is likely to act onanother molecule in terms of a molecular structure and an acceptorproperty is high.

Note that as the phthalocyanine-based materials mentioned above, aphthalocyanine-based material having a phenoxy group is preferable.Specifically, a phthalocyanine derivative having a phenoxy group, suchas PhO-VOPc, is preferable. The phthalocyanine derivative having aphenoxy group is soluble in a solvent and therefore has the advantage ofbeing easy to handle during formation of a light-emitting element andthe advantage of facilitating maintenance of an apparatus used for filmformation.

The electron-relay layer 707 may further contain a donor substance. Asthe donor substance, an organic compound such as tetrathianaphthacene(abbreviation: TTN), nickelocene, or decamethylnickelocene can be usedas well as an alkali metal, an alkaline earth metal, a rare earth metal,and a compound of the above metal (e.g., an alkali metal compound(including an oxide such as lithium oxide, a halide, and a carbonatesuch as lithium carbonate or cesium carbonate), an alkaline earth metalcompound (including an oxide, a halide, and a carbonate), and a rareearth metal compound (including an oxide, a halide, and a carbonate)).When such a donor substance is contained in the electron-relay layer707, electrons can be transferred easily and the light-emitting elementcan be driven at lower voltage.

In the case where a donor substance is contained in the electron-relaylayer 707, other than the materials given above as the substance havinga high electron-transport property, a substance having a LUMO levelhigher than the acceptor level of the acceptor substance contained inthe composite material layer 708 can be used. Specifically, it ispreferable to use a substance having a LUMO level higher than or equalto −5.0 eV, preferably higher than or equal to −5.0 eV and lower than orequal to −3.0 eV. As examples of such a substance, a perylenederivative, a nitrogen-containing condensed aromatic compound, and thelike are given. Note that a nitrogen-containing condensed aromaticcompound is preferably used for the electron-relay layer 707 because ofits high stability.

Specific examples of the perylene derivative are3,4,9,10-perylenetetracarboxylic dianhydride (abbreviation: PTCDA),3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (abbreviation:PTCBI), N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide(abbreviation: PTCDI-C8H), N,N′-dihexyl-3,4,9,10-perylenetetracarboxylicdiimide (abbreviation: Hex PTC), and the like.

Specific examples of the nitrogen-containing condensed aromatic compoundare pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile(abbreviation: PPDN),2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT(CN)₆), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation: 2PYPR),2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (abbreviation: F2PYPR), andthe like.

Besides, 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ),1,4,5,8-naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA),perfluoropentacene, copper hexadecafluorophthalocyanine (abbreviation:F₁₆CuPc),N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-1,4,5,8-naphthalenetetracarboxylic diimide (abbreviation: NTCDI-C8F),3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene(abbreviation: DCMT), methanofullerenes (e.g., [6,6]-phenyl C₆₁ butyricacid methyl ester), or the like can be used.

Note that in the case where a donor substance is contained in theelectron-relay layer 707, the electron-relay layer 707 may be formed bya method such as co-evaporation of the substance having a highelectron-transport property and the donor substance.

The hole-injection layer 701, the hole-transport layer 702, thelight-emitting layer 703, and the electron-transport layer 704 may eachbe formed using the above-described materials.

As described above, the EL layer 102 of this embodiment can be formed.

In the above-described light-emitting element, a current flows due to apotential difference generated between the first electrode 103 and thesecond electrode 108 and holes and electrons recombine in the EL layer102, so that light is emitted. Then, this light emission is extracted tothe outside through either the first electrode 103 or the secondelectrode 108 or both. Therefore, either the first electrode 103 or thesecond electrode 108, or both, is an electrode having a property oftransmitting visible light.

Note that the structure of layers provided between the first electrode103 and the second electrode 108 is not limited to the above-describedstructure. A structure other than the above may alternatively beemployed as long as a light-emitting region in which holes and electronsrecombine is provided in a portion away from the first electrode 103 andthe second electrode 108 so as to prevent quenching due to proximity ofthe light-emitting region to metal.

In other words, there is no particular limitation on a stack structureof the layers. A layer including a substance having a highelectron-transport property, a substance having a high hole-transportproperty, a substance having a high electron-injection property, asubstance having a high hole-injection property, a bipolar substance (asubstance having a high electron-transport property and a highhole-transport property), a hole-blocking material, or the like mayfreely be combined with a light-emitting layer.

In the above-described manner, the light-emitting element of oneembodiment of the present invention can be manufactured.

By the use of the light-emitting element described in this embodiment, apassive matrix light-emitting device or an active matrix light-emittingdevice in which driving of the light-emitting element is controlled by atransistor can be manufactured. Furthermore, the light-emitting devicecan be applied to an electronic device, a lighting device, or the like.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Example 1

In this example, an example of a combination of a first organiccompound, a second organic compound, and a phosphorescent compound whichcan be used for a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 1, FIG. 2,FIG. 3, and FIG. 4.

The phosphorescent compound used in Structure Examples 1 to 4 in thisexample is (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]). The first organic compound used inStructure Examples 1 to 4 in this example is2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II). As the second organic compound in this example,4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP) is used in Structure Example 1;3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), Structure Example 2;4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA or 1-TNATA), Structure Example 3; and2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF), Structure Example 4.

Chemical formulae of materials used in this example are shown below.

Structure Example 1

FIG. 1 shows an emission spectrum of a thin film of 2mDBTPDBq-II that isthe first organic compound (an emission spectrum 1a), an emissionspectrum of a thin film of PCBA1BP that is the second organic compound(an emission spectrum 2a), and an emission spectrum of a thin film of amixed material of 2mDBTPDBq-II and PCBA1BP (an emission spectrum 3a).Further, an ultraviolet-visible absorption spectrum (hereinafter, simplyreferred to as absorption spectrum) and an emission spectrum (anemission spectrum 4a) of [Ir(dppm)₂(acac)] that is the phosphorescentcompound in a dichloromethane solution of [Ir(dppm)₂(acac)] are alsoshown.

Note that in this example, the absorption spectrum of [Ir(dppm)₂(acac)]was measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

In FIG. 1, the horizontal axis represents wavelength (nm), and thevertical axes represent molar absorption coefficient ε (M⁻¹·cm⁻¹) andemission intensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 1, [Ir(dppm)₂(acac)]has a broad absorption band at around 510 nm. This absorption band isconsidered to greatly contribute to light emission.

The emission spectrum 3a peaks at a longer wavelength than the emissionspectra 1a and 2a. In addition, the peak of the emission spectrum 3a iscloser to the absorption band than the peaks of the emission spectra 1aand 2a. FIG. 1 shows that the emission spectrum 3a has the largestoverlap with the absorption band in the absorption spectrum whichgreatly contributes to light emission.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and PCBA1BP peaks at a longer wavelength than the emissionspectrum of either organic compound alone. This indicates that anexciplex is formed by mixing 2mDBTPDBq-II with PCBA1BP.

It is found that the peak of the emission spectrum 3a has a largeoverlap with the absorption band in the absorption spectrum of[Ir(dppm)₂(acac)] which is considered to greatly contribute to lightemission. Thus, it is indicated that the light-emitting elementincluding [Ir(dppm)₂(acac)] and the mixed material of 2mDBTPDBq-II andPCBA1BP has particularly high energy transfer efficiency because ittransfers energy by utilizing the large overlap between the emissionspectrum of the mixed material and the absorption spectrum of thephosphorescent compound. Accordingly, it is indicated that alight-emitting element having particularly high external quantumefficiency can be obtained.

Further, the peak of the emission spectrum 3a is located on the longerwavelength side as compared to the peak of the absorption spectrum andis located on a shorter wavelength side as compared to the peak of theemission spectrum 4a.

From the emission spectrum of the mixed material whose peak is locatedon the longer wavelength side, it is indicated that a light-emittingelement having low drive voltage can be obtained with the use of themixed material.

Structure Example 2

FIG. 2 shows an emission spectrum of a thin film of 2mDBTPDBq-II that isthe first organic compound (an emission spectrum 1b), an emissionspectrum of a thin film of PCzPCN1 that is the second organic compound(an emission spectrum 2b), and an emission spectrum of a thin film of amixed material of 2mDBTPDBq-II and PCzPCN1 (an emission spectrum 3b).Further, an absorption spectrum and an emission spectrum (an emissionspectrum 4b) of [Ir(dppm)₂(acac)] that is the phosphorescent compound ina dichloromethane solution of [Ir(dppm)₂(acac)] are also shown.

In FIG. 2, the horizontal axis represents wavelength (nm), and thevertical axes represent molar absorption coefficient ε (M⁻¹·cm⁻¹) andemission intensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 2, [Ir(dppm)₂(acac)]has a broad absorption band at around 510 nm. This absorption band isconsidered to greatly contribute to light emission.

The emission spectrum 3b peaks at a longer wavelength than the emissionspectra 1b and 2b. That is, it is found that the emission spectrum ofthe mixed material of 2mDBTPDBq-II and PCzPCN1 peaks at a longerwavelength than the emission spectrum of either organic compound alone.This indicates that an exciplex is formed by mixing 2mDBTPDBq-II withPCzPCN1.

Further, the peak of the emission spectrum 3b has an overlap with theabsorption spectrum of [Ir(dppm)₂(acac)]. Thus, it is indicated that thelight-emitting element including [Ir(dppm)₂(acac)] and the mixedmaterial of 2mDBTPDBq-II and PCzPCN1 has high energy transfer efficiencybecause it transfers energy by utilizing the overlap between theemission spectrum of the mixed material and the absorption spectrum ofthe phosphorescent compound. Accordingly, it is indicated that alight-emitting element having high external quantum efficiency can beobtained.

Further, the peak of the emission spectrum 3b is located on the longerwavelength side as compared to the peak of the absorption spectrum andis located on a shorter wavelength side as compared to the peak of theemission spectrum 4b. In addition, the difference between the peak ofthe emission spectrum 3b and that of the emission spectrum 4b is 21 nm,which is very small.

From the emission spectrum of the mixed material whose peak is locatedon the particularly long wavelength side, it is indicated that alight-emitting element having particularly low drive voltage can beobtained with the use of the mixed material.

Structure Example 3

FIG. 3 shows an emission spectrum of a thin film of 2mDBTPDBq-II that isthe first organic compound (an emission spectrum 1c), an emissionspectrum of a thin film of 1′-TNATA that is the second organic compound(an emission spectrum 2c), and an emission spectrum of a thin film of amixed material of 2mDBTPDBq-II and 1′-TNATA (an emission spectrum 3c).Further, an absorption spectrum and an emission spectrum (an emissionspectrum 4c) of [Ir(dppm)₂(acac)] that is the phosphorescent compound ina dichloromethane solution of [Ir(dppm)₂(acac)] are also shown.

In FIG. 3, the horizontal axis represents wavelength (nm), and thevertical axes represent molar absorption coefficient ε (M⁻¹·cm⁻¹) andemission intensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 3, [Ir(dppm)₂(acac)]has a broad absorption band at around 510 nm. This absorption band isconsidered to greatly contribute to light emission.

The emission spectrum 3c peaks at a longer wavelength than the emissionspectra 1c and 2c. That is, it is found that the emission spectrum ofthe mixed material of 2mDBTPDBq-II and 1′-TNATA peaks at a longerwavelength than the emission spectrum of either organic compound alone.This indicates that an exciplex is formed by mixing 2mDBTPDBq-II with1′-TNATA.

Further, it is found that the peak of the emission spectrum 3c has anoverlap with the absorption spectrum of [Ir(dppm)₂(acac)]. Thus, it isindicated that the light-emitting element including [Ir(dppm)₂(acac)]and the mixed material of 2mDBTPDBq-II and 1′-TNATA has high energytransfer efficiency because it transfers energy by utilizing the overlapbetween the emission spectrum of the mixed material and the absorptionspectrum of the phosphorescent compound. Accordingly, it is indicatedthat a light-emitting element having high external quantum efficiencycan be obtained.

Further, the peak of the emission spectrum 3c is located on the longerwavelength side as compared to the peak of the absorption spectrum. Inaddition, the difference between the peak of the emission spectrum 3cand that of the emission spectrum 4c is 24 nm, which is very small.

It can be considered that in the light-emitting element using the mixedmaterial, a value of voltage with which an exciplex is formed throughcarrier recombination is smaller than a value of voltage with which thephosphorescent compound starts to emit light by carrier recombination.In other words, even when voltage that has a value smaller than that ofvoltage with which the phosphorescent compound starts to emit light isapplied to the light-emitting element, carrier recombination occurs toform an exciplex and thus, current starts to flow in the light-emittingelement. Thus, it is indicated that a light-emitting element havingparticularly low drive voltage can be obtained.

Structure Example 4

FIG. 4 shows an emission spectrum of a thin film of 2mDBTPDBq-II that isthe first organic compound (an emission spectrum 1d), an emissionspectrum of a thin film of DPA2SF that is the second organic compound(an emission spectrum 2d), and an emission spectrum of a thin film of amixed material of 2mDBTPDBq-II and DPA2SF (an emission spectrum 3d).Further, an absorption spectrum and an emission spectrum (an emissionspectrum 4d) of [Ir(dppm)₂(acac)] that is the phosphorescent compound ina dichloromethane solution of [Ir(dppm)₂(acac)] are also shown.

In FIG. 4, the horizontal axis represents wavelength (nm), and thevertical axes represent molar absorption coefficient ε (M⁻¹·cm⁻¹) andemission intensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 4, [Ir(dppm)₂(acac)]has a broad absorption band at around 510 nm. This absorption band isconsidered to greatly contribute to light emission.

The emission spectrum 3d peaks at a longer wavelength than the emissionspectra 1d and 2d. That is, it is found that the emission spectrum ofthe mixed material of 2mDBTPDBq-II and DPA2SF peaks at a longerwavelength than the emission spectrum of either organic compound alone.This indicates that an exciplex is formed by mixing 2mDBTPDBq-II withDPA2SF.

Further, it is found that the peak of the emission spectrum 3d has anoverlap with the absorption spectrum of [Ir(dppm)₂(acac)]. Thus, it isindicated that the light-emitting element including [Ir(dppm)₂(acac)]and the mixed material of 2mDBTPDBq-II and DPA2SF has high energytransfer efficiency because it transfers energy by utilizing the overlapbetween the emission spectrum of the mixed material and the absorptionspectrum of the phosphorescent compound. Accordingly, it is indicatedthat a light-emitting element having high external quantum efficiencycan be obtained.

Further, the peak of the emission spectrum 3d is located on the longerwavelength side as compared to the peak of the absorption spectrum. Inaddition, the difference between the peak of the emission spectrum 3dand that of the emission spectrum 4d is 13 nm, which is very small.

From the emission spectrum of the mixed material whose peak is locatedon at the particularly long wavelength side, it is indicated that alight-emitting element having particularly low drive voltage can beobtained with the use of the mixed material.

Example 2

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 8. Chemicalformulae of materials used in this example are shown below. Note thatthe chemical formulae of the materials used in the above examples areomitted here.

Methods for manufacturing light-emitting elements 1 to 4 of this examplewill be described below.

(Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. Note that thethickness was set to 110 nm and the electrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, 4,4′,4″-(1,3,5-benzenetriyl)tri(dibenzothiophene)(abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and themass ratio of DBT3P-II to molybdenum oxide was adjusted to 1:0.5(=DBT3P-II:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of PCBA1BP was formedto a thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, PCBA1BP, and [Ir(dppm)₂(acac)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II toPCBA1BP and [Ir(dppm)₂(acac)] was adjusted to 0.7:0.3:0.05(=2mDBTPDBq-II:PCBA1BP:[Ir(dppm)₂(acac)]). The thickness of thelight-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 10 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film ofbathophenanthroline (abbreviation: BPhen) was formed to a thickness of20 nm to form a second electron-transport layer 1114 b.

Further, over the second electron-transport layer 1114 b, a film oflithium fluoride (LiF) was formed by evaporation to a thickness of 1 nmto form an electron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 1 of this example was fabricated.

(Light-Emitting Element 2)

The hole-transport layer 1112 of the light-emitting element 2 was formedby forming a film of PCzPCN1 to a thickness of 20 nm.

The light-emitting layer 1113 of the light-emitting element 2 was formedby co-evaporating 2mDBTPDBq-II, PCzPCN1, and [Ir(dppm)₂(acac)]. Here,the weight ratio of 2mDBTPDBq-II to PCzPCN1 and [Ir(dppm)₂(acac)] wasadjusted to 0.7:0.3:0.05 (=2mDBTPDBq-II:PCzPCN1:[Ir(dppm)₂(acac)]). Thethickness of the light-emitting layer 1113 was set to 40 nm. Componentsother than the light-emitting layer 1113 were manufactured in a mannersimilar to that of the light-emitting element 1.

(Light-Emitting Element 3)

The hole-transport layer 1112 of the light-emitting element 3 was formedby forming a film of 1′-TNATA to a thickness of 20 nm.

The light-emitting layer 1113 of the light-emitting element 3 was formedby co-evaporating 2mDBTPDBq-II, 1′-TNATA, and [Ir(dppm)₂(acac)]. Here,the weight ratio of 2mDBTPDBq-II to 1′-TNATA and [Ir(dppm)₂(acac)] wasadjusted to 0.7:0.3:0.05 (=2mDBTPDBq-II:1′-TNATA:[Ir(dppm)₂(acac)]). Thethickness of the light-emitting layer 1113 was set to 40 nm. Componentsother than the light-emitting layer 1113 were manufactured in a mannersimilar to that of the light-emitting element 1.

(Light-Emitting Element 4)

The hole-transport layer 1112 of the light-emitting element 4 was formedby forming a film of DPA2SF to a thickness of 20 nm.

The light-emitting layer 1113 of the light-emitting element 4 was formedby co-evaporating 2mDBTPDBq-II, DPA2SF, and [Ir(dppm)₂(acac)]. Here, theweight ratio of 2mDBTPDBq-II to DPA2SF and [Ir(dppm)₂(acac)] wasadjusted to 0.7:0.3:0.05 (=2mDBTPDBq-II:DPA2SF:[Ir(dppm)₂(acac)]). Thethickness of the light-emitting layer 1113 was set to 40 nm. Componentsother than the light-emitting layer 1113 were manufactured in a mannersimilar to that of the light-emitting element 1.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 1 shows element structures of the light-emitting elements 1 to 4obtained as described above.

TABLE 1 Second Hole- Hole- Electon- Electron- First injection transportFirst Electon- transport injection Second Electrode Layer LayerLight-emitting Layer transport Layer Layer Layer Electrode Light- ITSODBT3P- PCBA1BP 2mDBTPDBq-II:PCBA1BP:[Ir(dppm)₂(acac)] 2mDBTPDBq- BPhenLiF Al emitting 110 nm II:MoOx 20 nm (=0.7:0.3:0.05) II 20 nm 1 nm 200nm Element 1 (=1:0.5) 40 nm 10 nm 40 nm Light- ITSO DBT3P- PCzPCN12mDBTPDBq-II:PCzPCN1:[Ir(dppm)₂(acac)] 2mDBTPDBq- BPhen LiF Al emitting110 nm II:MoOx 20 nm (=0.7:0.3:0.05) II 20 nm 1 nm 200 nm Element 2(=1:0.5) 40 nm 10 nm 40 nm Light- ITSO DBT3P- 1′-TNATA2mDBTPDBq-II:1′-TNATA:[Ir(dppm)₂(acac)] 2mDBTPDBq- BPhen LiF Al emitting110 nm II:MoOx 20 nm (=0.7:0.3:0.05) II 20 nm 1 nm 200 nm Element 3(=1:0.5) 40 nm 10 nm 40 nm Light- ITSO DBT3P- DPA2SF2mDBTPDBq-II:DPA2SF:[Ir(dppm)₂(acac)] 2mDBTPDBq- BPhen LiF Al emitting110 nm II:MoOx 20 nm (=0.7:0.3:0.05) II 20 nm 1 nm 200 nm Element 4(=1:0.5) 40 nm 10 nm 40 nm

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 9 shows voltage-luminance characteristics of the light-emittingelements 1 to 4. In FIG. 9, the horizontal axis represents voltage (V),and the vertical axis represents luminance (cd/m²). Further, FIG. 10shows voltage-current characteristics. In FIG. 10, the horizontal axisrepresents voltage (V), and the vertical axis represents current (mA).FIG. 11 shows luminance-power efficiency characteristics thereof. InFIG. 11, the horizontal axis represents luminance (cd/m²), and thevertical axis represents power efficiency (lm/W). FIG. 12 showsluminance-external quantum efficiency characteristics thereof. In FIG.12, the horizontal axis represents luminance (cd/m²), and the verticalaxis represents external quantum efficiency (%).

Further, Table 2 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting elements 1 to 4 at a luminance of around 1000 cd/m².

TABLE 2 External Current Current Power Quantum Voltage DensityChromaticity Luminance Efficiency Efficiency Efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 2.7 1.5 (0.56, 0.44) 1000 6879 27 Element 1 Light-emitting 2.5 1.6 (0.56, 0.43) 1000 67 84 26Element 2 Light-emitting 2.7 2.3 (0.56, 0.43) 780 34 39 14 Element 3Light-emitting 2.4 1.3 (0.56, 0.43) 800 62 81 25 Element 4

FIG. 13 shows emission spectra of the light-emitting elements 1 to 4which were obtained by applying a current of 0.1 mA. In FIG. 13, thehorizontal axis represents wavelength (nm), and the vertical axisrepresents emission intensity (arbitrary unit). As shown in Table 2, theCIE chromaticity coordinates of the light-emitting element 1 at aluminance of 1000 cd/m² were (x, y)=(0.56, 0.44); the CIE chromaticitycoordinates of the light-emitting element 2 at a luminance of 1000 cd/m²were (x, y)=(0.56, 0.43); the CIE chromaticity coordinates of thelight-emitting element 3 at a luminance of 780 cd/m² were (x, y)=(0.56,0.43); and the CIE chromaticity coordinates of the light-emittingelement 4 at a luminance of 800 cd/m² were (x, y)=(0.56, 0.43). Theseresults show that orange light emission originating from[Ir(dppm)₂(acac)] was obtained from the light-emitting elements 1 to 4.

As can be seen from Table 2, FIG. 11, and FIG. 12, the light-emittingelements 1 to 4 have high current efficiency, high power efficiency, andhigh external quantum efficiency.

In the light-emitting elements of this example, the first organiccompound, the second organic compound, and the guest material which aredescribed in Example 1 are used for the light-emitting layer. Asdescribed in Example 1, the emission spectrum of the mixed material of2mDBTPDBq-II and the second organic compound (the emission spectrum ofan exciplex) overlaps with the absorption spectrum of [Ir(dppm)₂(acac)].The light-emitting elements of this example are considered to have highenergy transfer efficiency because it transfers energy by utilizing theoverlap, and therefore have high external quantum efficiency.

In this example, the light-emitting elements 1, 2, and 4 have higherexternal quantum efficiency than the light-emitting element 3 (see FIG.12). The reason for this is considered that the light-emitting elements1, 2, and 4 each have a larger overlap between the emission spectrum ofthe exciplex and the absorption spectrum of [Ir(dppm)₂(acac)] than thelight-emitting element 3 (refer to FIG. 1, FIG. 2, FIG. 3, FIG. 4, andExample 1).

As can be seen from FIG. 9 and FIG. 10, the light-emitting elements 1 to4 have low emission start voltage. The theoretical value of emissionstart voltage of an orange emissive organic EL element is said to beapproximately 2.1 V, to which the emission start voltage of thelight-emitting element of one embodiment of the present invention wasfound to be extremely close.

In this example, the light-emitting elements 2 to 4 have lower emissionstart voltage than the light-emitting element 1 (see FIG. 10 and FIG.11). The reason for this is considered that the emission spectrum of theexciplex of each of the light-emitting elements 2 to 4 peaks at a longerwavelength than the emission spectrum of the exciplex of thelight-emitting element 1 (refer to FIG. 1, FIG. 2, FIG. 3, FIG. 4, andExample 1).

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention. Moreover, it is shown that an element having lowdrive voltage can be obtained by application of one embodiment of thepresent invention.

Next, the light-emitting element 4 was subjected to reliability tests.Results of the reliability tests are shown in FIG. 14. In FIG. 14, thevertical axis represents normalized luminance (%) with an initialluminance of 100%, and the horizontal axis represents driving time (h)of the element.

In the reliability tests, the light-emitting element 4 was driven underthe conditions where the initial luminance was set to 5000 cd/m² and thecurrent density was constant.

The luminance of the light-emitting element 4 after 260 hours was 93% ofthe initial luminance. From the results, the light-emitting element 4 isfound to have a long lifetime.

The above results show that an element having low drive voltage and highreliability can be obtained by application of one embodiment of thepresent invention.

Example 3

In this example, a light-emitting element of one embodiment of thepresent invention will be described. Chemical formulae of the materialsused in this example are shown below. Note that the chemical formulae ofthe materials used in the above examples are omitted here.

Light-emitting elements formed in this example are Structure Examples ato s. An element structure of the Structure Examples a to s is shown inTable 3. Note that the Structure Examples a to s are different from oneanother in a substance X used in hole-transport layers andlight-emitting layers. Names of the substances X used in the structureexamples are shown below. Further, Table 4 shows the HOMO levels (eV) ofthe substances X used in the structure examples and emission peakwavelengths (nm) of exciplexes formed in the structure examples. Notethat in this example, a photoelectron spectrometer (AC-2, product ofRiken Keiki Co., Ltd.) was used to measure the HOMO level.

TABLE 3 Second Hole- Electon- Electron- First Hole-injection transportFirst Electon- transport injection Second Electrode Layer LayerLight-emitting Layer transport Layer Layer Layer Electrode Light- ITSODBT3P- Substance X 2mDBTPDBq-II:Substance 2mDBTPDBq-II BPhen LiF Alemitting 110 nm II:MoOx 20 nm X:[Ir(dppm)₂(acac)] 10 nm 20 nm 1 nm 200nm Element (=1:0.5) (=0.7:0.3:0.05) 40 nm 40 nm

TABLE 4 Peak Wavelength of HOMO Level of Emission Spectrum of StructureSubstance X Exciplex Example Substance X (eV) (nm) a PCBA1BP −5.42 519 bPCA2B −5.40 546 c DPNF −5.35 555 d PCA3B −5.31 553 e PCASF −5.30 543 fDPASF −5.30 571 g YGA2F −5.27 540 h TPD −5.25 537 i DPAB −5.23 573 jDFLADFL −5.20 557 k PCzPCA1 −5.17 571 l PCzDPA1 −5.16 581 m PCzDPA2−5.16 586 n PCzPCN1 −5.15 571 o DNTPD −5.14 573 p PCzTPN2 −5.13 582 qDPA2SF −5.09 579 r 1′-TNATA −5.09 616 s PCzPCA2 −5.08 575

Structure Example a

The Structure Example a is the light-emitting element 1 described inExample 2. PCBA1BP was used as the substance X.

Structure Example b

N,N′-bis(9-phenylcarbazol-3-yl)-N,N-diphenylbenzene-1,3-diamine(abbreviation: PCA2B) was used as the substance X.

Structure Example c

As the substance X was usedN-(9,9-dimethyl-2-N,N′-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF).

Structure Example d

N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B) was used as the substance X.

Structure Example e

As the substance X was used2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF).

Structure Example f

As the substance X was used2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF).

Structure Example g

N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F) was used as the substance X.

Structure Example h

As the substance X was usedN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD).

Structure Example i

As the substance X was used4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB).

Structure Example j

N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL) was used as the substance X.

Structure Example k

As the substance X was used3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1).

Structure Example 1

As the substance X was used3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1).

Structure Example m

As the substance X was used3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2).

Structure Example n

The Structure Example n is the light-emitting element 2 described inExample 2. PCzPCN1 was used as the substance X.

Structure Example o

As the substance X was used4,4′-bis(N-{4-[N′-(3-methylphenyl)-N-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD).

Structure Example p

As the substance X was used3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2).

Structure Example q

The Structure Example q is the light-emitting element 4 described inExample 2. DPA2SF was used as the substance X.

Structure Example r

The Structure Example r is the light-emitting element 3 described inExample 2. As the substance X, 1′-TNATA was used.

Structure Example s

As the substance X was used3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2).

FIG. 15 shows a relationship between the peak wavelength of the emissionspectrum of the exciplex and the HOMO level of the substance X in eachstructure example. In FIG. 15, the horizontal axis represents the peakwavelength (nm) and the vertical axis represents the HOMO level (eV).Further, FIG. 16 shows a relationship between the peak wavelength of theemission spectrum of the exciplex and relative external quantumefficiency in each structure example. In FIG. 16, the horizontal axisrepresents the peak wavelength (nm) and the vertical axis represents therelative external quantum efficiency (arbitrary unit). Note that therelative external quantum efficiency in FIG. 16 is shown as a valuerelative to the external quantum efficiency of the light-emittingelement in the Structure Example a assumed to be 1. In FIG. 16, relativeexternal quantum efficiency of the Structure Example e and the StructureExample h is not shown.

It is indicated from FIG. 15 that when the HOMO level of the substance Xis higher, the emission spectrum of the exciplex formed from2mDBTPDBq-II and the substance X peaks at a longer wavelength.Therefore, when the HOMO level of the substance X is higher and theemission spectrum of the exciplex peaks at a longer wavelength, theemission start voltage of the light-emitting element can be lower.Accordingly, the light-emitting element can have lower drive voltage.

From FIG. 16, it can be seen that the external quantum efficiency of thelight-emitting element is low when the peak wavelength of the emissionspectrum of the exciplex is too long. What is indicated is that the peakwavelength of the emission spectrum of the exciplex has a preferablerange for high external quantum efficiency and low drive voltage of alight-emitting element. Specifically, it is indicated that in order thatboth low drive voltage and high external quantum efficiency be achieved,the peak wavelength of the emission spectrum of the exciplex ispreferably longer than or equal to the peak wavelength (in this example,approximately 510 nm in the absorption spectrum of the phosphorescentcompound in a solution) of the absorption band located on the longestwavelength side of the absorption spectrum of the phosphorescentcompound, and preferably shorter than or equal to the peak wavelength(in this example, approximately 580 nm in an emission spectrum ofelectroluminescence) of the emission spectrum of the phosphorescentcompound.

Reference Example 1

A synthetic example of an organometallic complex(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (anothername:bis[2-(6-phenyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III))(abbreviation: [Ir(dppm)₂(acac)]), which is used in the above examples,is described.

The structure of [Ir(dppm)₂(acac)] is shown below.

Step 1: Synthesis of 4,6-Diphenylpyrimidine (abbreviation: Hdppm)

First, 5.02 g of 4,6-dichloropyrimidine, 8.29 g of phenylboronic acid,7.19 g of sodium carbonate, 0.29 g ofbis(triphenylphosphine)palladium(II)dichloride (abbreviation:Pd(PPh₃)₂Cl₂), 20 mL of water, and 20 mL of acetonitrile were put into arecovery flask equipped with a reflux pipe, and the air in the flask wasreplaced with argon. This reaction container was heated by irradiationwith microwaves (2.45 GHz, 100 W) for 60 minutes. Here, there werefurther put 2.08 g of phenylboronic acid, 1.79 g of sodium carbonate,0.070 g of Pd(PPh₃)₂Cl₂, 5 mL of water, and 5 mL of acetonitrile intothe flask, and the mixture was heated again by irradiation withmicrowaves (2.45 GHz, 100 W) for 60 minutes. After that, water was addedto this solution and an organic layer was extracted withdichloromethane. The obtained solution of the extract was washed withwater and dried with magnesium sulfate. The solution after drying wasfiltered. The solvent of this solution was distilled off, and then theobtained residue was purified by silica gel column chromatography usingdichloromethane as a developing solvent, so that a pyrimidine derivativeHdppm (yellow white powder, yield of 38%) was obtained. Note that forthe irradiation with microwaves, a microwave synthesis system (Discover,manufactured by CEM Corporation) was used. A synthesis scheme (a-1) ofStep 1 is shown below.

Step 2: Synthesis ofDi-μ-chloro-bis[bis(4,6-diphenylpyrimidinato)iridium(III)](abbreviation:[Ir(dppm)₂Cl]₂)

Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.10 g of Hdppm obtainedin Step 1, and 0.69 g of iridium chloride hydrate (IrCl₃.H₂O) were putinto a recovery flask equipped with a reflux pipe, and the air in therecovery flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 100 W) was performed for 1 hour to cause areaction. The solvent was distilled off, and then the obtained residuewas filtered and washed with ethanol to give a dinuclear complex[Ir(dppm)₂Cl]₂ (reddish brown powder, yield of 88%). A synthesis scheme(a-2) of Step 2 is shown below.

Step 3: Synthesis of(Acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)])

Furthermore, 40 mL of 2-ethoxyethanol, 1.44 g of [Ir(dppm)₂Cl]₂ obtainedin Step 2, 0.30 g of acetylacetone, and 1.07 g of sodium carbonate wereput into a recovery flask equipped with a reflux pipe, and the air inthe recovery flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 120 W) was performed for 60 minutes to cause areaction. The solvent was distilled off, the obtained residue wasdissolved in dichloromethane, and filtration was performed to removeinsoluble matter. The obtained filtrate was washed with water and thenwith saturated saline, and was dried with magnesium sulfate. Thesolution after drying was filtered. The solvent of this solution wasdistilled off, and then the obtained residue was purified by silica gelcolumn chromatography using dichloromethane and ethyl acetate as adeveloping solvent in a volume ratio of 50:1. After that,recrystallization was carried out with a mixed solvent ofdichloromethane and hexane, so that the objective orange powder (yieldof 32%) was obtained. A synthesis scheme (a-3) of Step 3 is shown below.

An analysis result by nuclear magnetic resonance spectrometry (¹H NMR)of the orange powder obtained in Step 3 is described below. Theseresults revealed that the organometallic complex [Ir(dppm)₂(acac)] wasobtained.

¹H NMR. δ (CDCl₃): 1.83 (s, 6H), 5.29 (s, 1H), 6.48 (d, 2H), 6.80 (t,2H), 6.90 (t, 2H), 7.55-7.63 (m, 6H), 7.77 (d, 2H), 8.17 (s, 2H), 8.24(d, 4H), 9.17 (s, 2H).

Reference Example 2

A method for synthesizing2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II), which is used in the above examples, is described.

Synthesis of 2-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTPDBq-II))

A synthesis scheme (b−1) of2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) is shown below.

First, 5.3 g (20 mmol) of 2-chlorodibenzo[f,h]quinoxaline, 6.1 g (20mmol) of 3-(dibenzothiophen-4-yl)phenylboronic acid, 460 mg (0.4 mmol)of tetrakis(triphenylphosphine)palladium(0), 300 mL of toluene, 20 mL ofethanol, and 20 mL of a 2M aqueous solution of potassium carbonate wereput in a 2 L three-neck flask. The mixture was degassed by being stirredunder reduced pressure, and the air in the three-neck flask was replacedwith nitrogen. This mixture was stirred under a nitrogen stream at 100°C. for 7.5 hours. After cooled to room temperature, the obtained mixturewas filtered to give a white residue. The obtained residue was washedwith water and ethanol in this order, and then dried. The obtained solidwas dissolved in about 600 mL of hot toluene, followed by suctionfiltration through Celite (produced by Wako Pure Chemical Industries,Ltd., Catalog No. 531-16855) and Florisil (produced by Wako PureChemical Industries, Ltd., Catalog No. 540-00135), whereby a clearcolorless filtrate was obtained. The obtained filtrate was concentratedand purified by silica gel column chromatography using about 700 mL ofsilica gel. The chromatography was carried out using hot toluene as adeveloping solvent. Acetone and ethanol were added to the solid obtainedhere, followed by irradiation with ultrasonic waves. Then, the generatedsuspended solid was collected by filtration and the obtained solid wasdried, so that 7.85 g of white powder was obtained in 80% yield.

The above objective substance was relatively soluble in hot toluene, butwas a material that was likely to be precipitated when cooled. Further,the substance was poorly soluble in other organic solvents such asacetone and ethanol. Hence, the utilization of these different degreesof solubility resulted in a high-yield synthesis by a simple method asabove. Specifically, after the reaction finished, the mixture wasreturned to room temperature and the precipitated solid was collected byfiltration, whereby most impurities were able to be easily removed.Further, by the column chromatography with hot toluene as a developingsolvent, the objective substance, which is likely to be precipitated,was able to be readily purified.

By a train sublimation method, 4.0 g of the obtained white powder waspurified. In the purification, the white powder was heated at 300° C.under a pressure of 5.0 Pa with a flow rate of argon gas of 5 mL/min.After the purification, the objective substance was obtained in a yieldof 88% as 3.5 g of white powder.

A nuclear magnetic resonance spectrometry (¹H NMR) identified thiscompound as the objective2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II).

¹H NMR data of the obtained substance are shown below.

¹H NMR (CDCl₃, 300 MHz): δ (ppm)=7.45-7.52 (m, 2H), 7.59-7.65 (m, 2H),7.71-7.91 (m, 7H), 8.20-8.25 (m, 2H), 8.41 (d, J=7.8 Hz, 1H), 8.65 (d,J=7.5 Hz, 2H), 8.77-8.78 (m, 1H), 9.23 (dd, J=7.2 Hz, 1.5 Hz, 1H), 9.42(dd, J=7.8 Hz, 1.5 Hz, 1H), 9.48 (s, 1H).

REFERENCE NUMERALS

-   102: EL layer, 103: first electrode, 108: second electrode, 701:    hole-injection layer, 702: hole-transport layer, 703: light-emitting    layer, 704: electron-transport layer, 705: electron-injection layer,    706: electron-injection buffer layer, 707: electron-relay layer,    708: composite material layer, 800: first EL layer, 801: second EL    layer, 803: charge generation layer, 1100: substrate, 1101: first    electrode, 1103: second electrode, 1111: hole-injection layer, 1112:    hole-transport layer, 1113: light-emitting layer, 1114 a: first    electron-transport layer, 1114 b: second electron-transport layer,    and 1115: electron-injection layer

This application is based on Japanese Patent Application serial no.2011-064553 filed with Japan Patent Office on Mar. 23, 2011, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A light-emitting element comprising: alight-emitting layer comprising a phosphorescent compound and a firstcompound; wherein the first compound is capable of forming an exciplex,and wherein an emission peak wavelength of the exciplex is longer thanor equal to a peak wavelength of an absorption of a triplet metal toligand charge transfer transition of the phosphorescent compound.
 2. Thelight-emitting element according to claim 1, wherein excitation energyof the exciplex is transferred to the phosphorescent compound so thatthe phosphorescent compound emits phosphorescence.
 3. The light-emittingelement according to claim 1, wherein a difference between the emissionpeak wavelength of the exciplex and the peak wavelength of theabsorption of the triplet metal to ligand charge transfer transition ofthe phosphorescent compound is 30 nm or less.
 4. The light-emittingelement according to claim 1, wherein the exciplex is formed from asinglet exciton of the first compound.
 5. The light-emitting elementaccording to claim 1, wherein the light-emitting layer further comprisesa second compound, and wherein the first compound and the secondcompound are configured to form the exciplex.
 6. The light-emittingelement according to claim 5, wherein the exciplex is formed from ananion of the first compound and a cation of the second compound.
 7. Thelight-emitting element according to claim 5, wherein at least one of thefirst compound and the second compound is a fluorescent compound.
 8. Thelight-emitting element according to claim 1, wherein the phosphorescentcompound is an organometallic complex.
 9. An electronic devicecomprising a light-emitting device comprising a transistor and thelight-emitting element according to claim
 1. 10. A lighting devicecomprising a light-emitting device comprising a light-emitting elementaccording to claim
 1. 11. A light-emitting element comprising: alight-emitting layer comprising a phosphorescent compound and a firstcompound; wherein the first compound is capable of forming an exciplex,wherein an emission peak wavelength of the exciplex is longer than orequal to a peak wavelength of an absorption of a triplet metal to ligandcharge transfer transition of the phosphorescent compound and shorterthan or equal to an emission peak wavelength of the phosphorescentcompound.
 12. The light-emitting element according to claim 11, whereinexcitation energy of the exciplex is transferred to the phosphorescentcompound so that the phosphorescent compound emits phosphorescence. 13.The light-emitting element according to claim 11, wherein a differencebetween the emission peak wavelength of the exciplex and the peakwavelength of the absorption of the triplet metal to ligand chargetransfer transition of the phosphorescent compound is 30 nm or less. 14.The light-emitting element according to claim 11, wherein the exciplexis formed from a singlet exciton of the first compound.
 15. Thelight-emitting element according to claim 11, wherein the light-emittinglayer further comprises a second compound, and wherein the firstcompound and the second compound are configured to form the exciplex.16. The light-emitting element according to claim 15, wherein theexciplex is formed from an anion of the first compound and a cation ofthe second compound.
 17. The light-emitting element according to claim15, wherein at least one of the first compound and the second compoundis a fluorescent compound.
 18. The light-emitting element according toclaim 11, wherein the phosphorescent compound is an organometalliccomplex.
 19. An electronic device comprising a light-emitting devicecomprising a transistor and the light-emitting element according toclaim
 11. 20. A lighting device comprising a light-emitting devicecomprising a light-emitting element according to claim 11.